Add PHP 7.4 to travis matrix

Author: bpolaszek

.travis.yml

@@ -1,18 +1,24 @@
 language: php
 
 php:
-  - '5.4'
-  - '5.5'
   - '5.6'
   - '7.0'
   - '7.1'
   - '7.2'
   - '7.3'
+  - '7.4'
+  - 'nightly'
 
 matrix:
   include:
     - php: '5.3'
       dist: precise
+    - php: '5.4'
+      dist: trusty
+    - php: '5.5'
+      dist: trusty
+  allow_failures:
+    - php: 'nightly'
 
 before_script:
   - travis_retry composer self-update