Travis: don't allow PHP 7.4 build to fail

As [PHP 7.4 has been released](https://www.php.net/archive/2019.php#2019-11-28-1), the build against PHP 7.4 should no longer be allowed to fail.

Includes fixing the `language` setting. This can't be an array and thrown warnings as it was when the Travis script is validated.

.travis.yml

@@ -8,8 +8,7 @@ cache:
     # Cache directory for more recent Composer versions.
     - $HOME/.cache/composer/files
 
-language:
-    - php
+language: php
 
 php:
     - 5.4
@@ -112,10 +111,6 @@ jobs:
     - php: 5.4
       env: PHPCS_BRANCH="3.3.1"
 
-  allow_failures:
-    # Allow failures for unstable builds.
-    - php: "7.4snapshot"
-
 before_install:
     # Speed up build time by disabling Xdebug.
     # https://johnblackbourn.com/reducing-travis-ci-build-times-for-wordpress-projects/