Update tests/React/Functional/Stomp/FunctionalTestCase.php

Fix error message

`realpath` would return null if the file does not exist
Merge remote-tracking branch 'romainneutron/patch-1'

* romainneutron/patch-1:
  Update tests/React/Functional/Stomp/FunctionalTestCase.php
Throw exception when invalid STOMP_PROVIDER is passed
Prepare v0.1.3 release