Update .travis.yml

Author: enumag

.travis.yml

@@ -16,14 +16,9 @@ script:
 
 matrix:
   include:
-      - php: 5.6
-        env: PHPUNIT=~5.7
       - php: 7.0
-        env: PHPUNIT=~5.7
+        env: PHPUNIT=~6.4
       - php: 7.1
-        env: PHPUNIT=~6
+        env: PHPUNIT=~7
       - php: nightly
-        env: PHPUNIT=~6
-      - php: hhvm
-        dist: trusty
-        env: PHPUNIT=~5.7
+        env: PHPUNIT=~7