Properly test on PHP 7.4 again (#180)

* Revert "allow PHP 7.4 failure given not available on worker(s) yet..."

This reverts commit e6f2737.

* Adjust PHPUnit versions

Author: mmoll

.travis.yml

@@ -23,7 +23,6 @@ script:
 
 matrix:
   allow_failures:
-    - php: 7.4
     - php: nightly
 
 notifications:

composer.json

@@ -28,7 +28,7 @@
         "squizlabs/php_codesniffer": "^3.3.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.0 || ^5.0 || ^6.0 || ^7.0"
+        "phpunit/phpunit": "^5.0 || ^6.0 || ^7.0"
     },
     "conflict": {
         "squizlabs/php_codesniffer": "<3 || >=4"