Update phpunit/phpunit requirement from ^6.0 to ^8.1

Updates the requirements on phpunit/phpunit to permit the latest version.

Changelog

*Sourced from [phpunit/phpunit's changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.1.md).* > ## [8.1.6] - 2019-05-28 > > ### Changed > > * After each test, `libxml_clear_errors()` is now called to clear the libxml error buffer > > ### Fixed > > * Fixed [#3694](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/pull/3694): Constructor arguments for `Throwable` and `Exception` are ignored > * Fixed [#3709](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/pull/3709): Method-level `@coversNothing` annotation does not prevent code coverage data collection > > ## [8.1.5] - 2019-05-14 > > ### Fixed > > * Fixed [#3683](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3683): Regression in PHPUnit 8.1.4 with regard to Exception stubbing/mocking > > ## [8.1.4] - 2019-05-09 > > ### Fixed > > * Fixed [#3414](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/pull/3414): `willThrowException()` only accepts `Exception`, not `Throwable` > * Fixed [#3559](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3559): No diff for failed PHPT EXPECT > * Fixed [#3587](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3587): `.phpunit.result.cache` file is all over the place > * Fixed [#3596](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3596): Mocking an interface that extends another interface forgets to mock its own methods > * Fixed [#3599](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3599): Type error in `TestCase::createGlobalStateSnapshot()` > * Fixed [#3614](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/pull/3599): `PHPUnit\Framework\Constraint\Attribute` should be deprecated (and ignored from code coverage) > * Fixed [#3674](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3674): `TypeError` when an incorrect file path is given > > ## [8.1.3] - 2019-04-19 > > ### Fixed > > * Fixed [#3607](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3607): Return value generation interferes with proxying to original method > > ## [8.1.2] - 2019-04-08 > > ### Fixed > > * Fixed [#3600](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/pull/3600): Wrong class name in docblock > > ## [8.1.1] - 2019-04-08 > > ### Fixed > > * Fixed [#3588](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3588): PHPUnit 8.1.0 breaks static analysis of MockObject usage > > ## [8.1.0] - 2019-04-05 > > ### Added > ... (truncated)

Commits

- [`e3c9da6`](https://github.com/sebastianbergmann/phpunit/commit/e3c9da6e645492c461e0a11eca117f83f4f4c840) Prepare release - [`f5e7714`](https://github.com/sebastianbergmann/phpunit/commit/f5e77146b36aafff100a66512df3313119e05d6b) Merge branch '7.5' into 8.1 - [`54880ea`](https://github.com/sebastianbergmann/phpunit/commit/54880ea49648d9a90c613f05fd734cd1d6e43eb4) Prepare release - [`4ab4f62`](https://github.com/sebastianbergmann/phpunit/commit/4ab4f62fc253e8a2d30b64950aa3443e2f8eaedc) Merge branch '7.5' into 8.1 - [`aadc181`](https://github.com/sebastianbergmann/phpunit/commit/aadc181a16be7f5cd24658dfe18f9a3d9d9c9e9e) Update ChangeLog - [`63a0a6c`](https://github.com/sebastianbergmann/phpunit/commit/63a0a6c5f8dd7eb5bebe85d4917f103e8868b436) Fix [#3697](https://github-redirect.dependabot.com/sebastianbergmann/phpunit/issues/3697) Respect [@coversNothing](https://github.com/coversNothing) at coverage driver start - [`5607fa4`](https://github.com/sebastianbergmann/phpunit/commit/5607fa4f196bf155a2905b06688c523e37072ed1) Specify test name when initing TestCase - [`9823dc2`](https://github.com/sebastianbergmann/phpunit/commit/9823dc2564e46013be35857572635a99c76f06e7) Merge branch '7.5' into 8.1 - [`1426ca6`](https://github.com/sebastianbergmann/phpunit/commit/1426ca6a2d3c5fc2afb77ac043ce34a11288f0b2) Remove AppVeyor configuration - [`510419a`](https://github.com/sebastianbergmann/phpunit/commit/510419aebaf83b361c8a20553c976b47a0e4fe62) Sync with 1d4dfbf6cd3a50aaceb7af96ba5dbb5b518f7434 - Additional commits viewable in [compare view](https://github.com/sebastianbergmann/phpunit/compare/6.0.0...8.1.6)

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cakephp / elastic-search

Update phpunit/phpunit requirement from ^6.0 to ^8.1 #203