New --api_version argument 

Allows overriding client API version to match server API version. 

Using this plugin with current docker-py (DEFAULT_DOCKER_API_VERSION = '1.16' from https://github.com/docker/docker-py/commit/7bc6b228b187b8de20b20e2d66518660d2cf5895) against docker daemon 1.3.3 (Server API version: 1.15) failed with the following error (after disabling exception handling):
```
DOCKER UNKNOWN: APIError: 404 Client Error: Not Found ("client and server don't have same version (client : 1.16, server: 1.15)")
```