Remove :url param from aws get_service request

As of geemus/excon@cb6b273 Excon validates params for ```request``` method.
However, array of valid params doesn’t include ```url``` param, which ends up with ```ArgumentError: The following keys are invalid: :url``` exception.
I removed that param from ```get_service``` method, and everything seems to be OK, however, I’m not sure if this is not excon’s bug, and I shouldn’t add ```:url``` to ```VALID_CONNECTION_KEYS``` array – if so, just let me know.