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

Author: jrfnl

.travis.yml

@@ -21,6 +21,7 @@ php:
     - 7.1
     - 7.2
     - 7.3
+    - "7.4snapshot"
     - nightly
 
 env:
@@ -42,6 +43,7 @@ matrix:
 
   allow_failures:
     # Allow failures for unstable builds.
+    - php: "7.4snapshot"
     - php: nightly
 
 before_install: