Bump PHP versions on Travis

Ubuntu Xenial only supports PHP 5.6+.

Author: jtojnar

.travis.yml

@@ -1,10 +1,12 @@
 language: php
 
 php:
-  - 5.4
-  - 5.5
   - 5.6
-  - hhvm-nightly
+  - 7.0
+  - 7.1
+  - 7.2
+  - 7.3
+  - 8.0
 
 before_script:
   - composer install --dev