Travis: test builds against PHP 7.4
Travis: test builds against PHP 7.4

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`.

Yoast/yoastcs