Docker can make requests to other API versions

Sometimes is useful to have the last version of this gem, but the target DOCKER_HOST could be outdated.

Setting the API version in the Docker:

````
Docker.api_version = "1.10"
Docker.version
````

will make the request to `/v1.10/version`. This will allow checking for older versions of the Docker daemon and either fail gracefully or use a reduced set of the API (for instance, no `pause`/`unpause` before v.1.12).

Currently, trying to talk to a outdated Docker daemon results in a non-informative error.