Fix Travis CI build matrix with PHP 7.4 and nightly builds as allowed_failures

Author: Ayesh

.travis.yml

@@ -1,9 +1,13 @@
 language: php
-php:
-  - 7.2
-  - 7.3
-  - 7.4
-  - nightly
+matrix:
+  include:
+    - php: 7.2
+    - php: 7.3
+    - php: 7.4
+    - php: nightly
+  allow_failures:
+    - php: 7.4
+    - php: nightly
 
 before_script:
   - composer self-update