Support API version 1.20

[shin-]

No description provided.

[aanand]

Those snippets look near-identical. Maybe there should be a test utility method?

def get_volume_info(container, destination, version):
    """
    Takes a container dictionary and volume destination path
    and returns a tuple of (rw, origin), using the appropriate
    fields for the specified API version
    """
    if docker.utils.compare_version('1.20', version) < 0:
        # old logic
    else:
        # new logic

[aanand]

This seems to be a duplicate of the line above - merge conflict resolution mishap?

[aanand]

Duplicate line aside, LGTM