Remove HHVM from Travis testing.

Only because vfsStream does not support HHVM. Plates should work
without issue on HHVM.

Author: reinink

.travis.yml

@@ -1,29 +1,21 @@
 language: php
 
-sudo: false
-
 php:
   - 5.3
   - 5.4
   - 5.5
   - 5.6
   - 7.0
   - 7.1
-  - hhvm
 
-matrix:
-  fast_finish: true
-  allow_failures:
-    - php: hhvm
-    
+sudo: false
+
 cache:
   directories:
     - $HOME/.composer/cache
 
 before_install:
   - travis_retry composer self-update
-
-install:
   - travis_retry composer install --no-interaction --prefer-source
 
 script: