connection error in SQLalchemy due to version string regex mismatch

version string regex in [`sqlalchemy._get_server_version_info`](https://github.com/cloudera/impyla/blob/master/impala/sqlalchemy.py#L121) did not fully capture Impala version string (e.g. 'cdh5-1.3.0'). CDH version prefix was not covered.

Please note: this is just a minimal fix. I think there should be some exception handling since the version extraction as is seems to be quite brittle.