Fix a compatibility issue with Guzzle

Declaration of FH\PostcodeAPIClient\FHPostcodeAPIClient::createRequest() must be compatible with Guzzle\Http\ClientInterface::createRequest($method = 'GET', $uri = NULL, $headers = NULL, $body = NULL, array $options = Array)
Merge pull request #1 from bryantebeek/patch-1

Fix a compatibility issue with Guzzle