Travis: test builds against PHP 7.4

[jrfnl]

Summary

This PR can be summarized in the following changelog entry:

• N/A

Relevant technical choices:

Nightly has become PHP 8.0 since PHP 7.4 has been branched, so to continue to also test against PHP 7.4, it needs to be added separately.

Refs:

• https://twitter.com/nikita_ppv/status/1089839541828112384
• https://twitter.com/nikita_ppv/status/1094897743594770433

As PHP 8.x - current nightly - is not expected to be released until December 2020, with PHP 7.4 expected in December 2019, I've elected to replace the build against nightly with a build against 7.4snapshot.

Once PHP 7.4 is released, the "high PHP" build - currently PHP 7.3 - should be replaced with a build against PHP 7.4 (stable) and the "unstable" build - now 7.4snapshot - should be reverted to nightly.

Test instructions

This PR can be tested by following these steps:

• Check the Travis build output.

[jrfnl]

Note: the PHP 7.4 build is currently failing. This is due to a PHP 7.4 related issue in the Mockery dependency and should be fixed there.

[Dieterrr]

CR done 👍