make travis use phpunit < 6 to avoid failing builds for PHP 7, see: https://github.com/travis-ci/travis-ci/issues/7226#issuecomment-285852088

.travis.yml

@@ -10,7 +10,7 @@ before_script:
   - composer install -n
 
 script:
-  - cd tst && phpunit
+  - cd tst && ../vendor/bin/phpunit
 
 after_script:
   - cd ..

composer.json

@@ -24,7 +24,8 @@
     },
     "require-dev": {
         "codacy/coverage": "dev-master",
-        "codeclimate/php-test-reporter": "dev-master"
+        "codeclimate/php-test-reporter": "dev-master",
+        "phpunit/phpunit": "^4.6 || ^5.0"
     },
     "autoload": {
         "psr-4": {