Fix: Run build against PHP 7.4

Author: localheinz

Diff for: .travis.yml

@@ -12,6 +12,8 @@ jobs:
     - php: 7.1
     - php: 7.2
     - php: 7.3
+    - php: 7.4
+      dist: bionic
       env: WITH_COVERAGE=true
     - php: nightly