try to find a phpunit version that is compatible with both PHP 5.5 and HHVM

.travis.yml

@@ -11,7 +11,7 @@ before_script:
   - php composer.phar install -n
 
 script:
-  - vendor/bin/phpunit tst
+  - cd tst && ../vendor/bin/phpunit
 
 after_script:
-  - php vendor/bin/codacycoverage clover tst/log/coverage-clover.xml
+  - vendor/bin/codacycoverage clover tst/log/coverage-clover.xml

composer.json

@@ -1,6 +1,6 @@
 {
     "require-dev": {
-        "phpunit/phpunit": "5.4.*",
+        "phpunit/phpunit": "4.8.*",
         "codacy/coverage": "dev-master"
     }
 }