Update .travis.yml

phpunit cannot support less than 5.6

Author: luxin88

.travis.yml

@@ -1,12 +1,9 @@
 language: php
 
 php:
-  - 5.3
-  - 5.4
-  - 5.5
   - 5.6
   - 7.0
-  - hhvm
+  - 7.1
 
 before_script:
   - composer self-update
@@ -17,5 +14,4 @@ script: phpunit
 matrix:
   allow_failures:
     - php: 7.0
-    - php: hhvm
-  fast_finish: true
+  fast_finish: true