Bump minimal version of "phpunit-bridge" (#111)

* Move "phpunit-bridge" to "require-dev" section

* Bump minimal version to 5.1

Author: jderusse

composer.json

@@ -26,7 +26,7 @@
         "php": ">=5.5.9",
         "psr/cache": "~1.0",
         "cache/tag-interop": "^1.0",
-        "symfony/phpunit-bridge": "^4.4|^5.0"
+        "symfony/phpunit-bridge": "^5.1"
     },
     "require-dev": {
         "cache/cache": "^1.0",