permit higher phpunit to be used. increases compatibility with new php versions (#13)

Author: davidbankes

composer.json

@@ -28,7 +28,7 @@
     "require": {
         "guzzle/plugin-mock": "~3.1",
         "mockery/mockery": "~0.8",
-        "phpunit/phpunit": "~3.7.0",
+        "phpunit/phpunit": "~3.7.0|~4.0",
         "squizlabs/php_codesniffer": "~1.5"
     },
     "extra": {