Drop unsupported language/package versions

[Jean85]

I would like to bump the supported versions for those deps with the 1.0 stable release:

• [x] PHPUnit ^6.0 (4.x drops support 2016-02-27, cross support is too hard)
• [x] Symfony ^2.7 || ^3.0
• [x] PHP ^7.0 (due to lack of support from PHPUnit 6)

This means also a few optimizations in the works:

• [x] support PHPUnit 6 (partially done with #86 )
• [x] use ::class everywhere
• [x] use array short syntax everywhere
• [x] use scalar and return typehint everywhere
• [x] use strict_types=1 everywhere
• [x] check for PHPUnit deprecations (setExpectedException, ...)
• [x] check for Symfony deprecations (maybe using symfony/phpunit-bridge)
• [x] drop Symfony/HttpKernel and the RegisterListenersPass workaround
• [x] disable skipping for RunnerTest::testWarning
• [x] remove check about presence of JSON_PRETTY_PRINT, it's there since 5.6.1
• [x] remove fallback versions of assertFile[Not]Exists
• [x] check that PHPUnit's options are correctly aligned with the 6.x version

[dxops]

PHP <5.6 (due to lack of support from PHPUnit 5)

It's supported up to PHPUnit 6, I'm not sure we need to drop it until PHP 5.6 EOL

[Jean85]

It's < (less than) 5.6, so I will drop up to 5.5, following PHPUnit.

[Jean85]

Since I'm stuck with #89 and #92, I think I will give this a go, maybe it will help...

[Jean85]

Trying to support both PHPUnit 5 and 6 is getting harder than it seemed. There are a lot of classes that do not have FC compatibility between those versions, and even those that have that, have issues (see https://github.com/sebastianbergmann/phpunit/pull/2685 for example).

I'm thinking about bumping to a minimum of PHP 7 and PHPUnit 6. That should be a lot easier and we should gain scalar and return typehint.

Also, PHP 5.6 has left active support 4 months ago, and Paraunit 0.8.x will still hang around if needed...

[Jean85]

After merging #93 and 286bf88 we can consider this done!