Drop support for API versions <1.24

Implemented as just a warning. Actual removal of code will follow
sometime in the future.
Signed-off-by: Ben Firshman <ben@firshman.co.uk>

docker/client.py

 import json
 import struct
+import warnings
 from functools import partial
 
 import requests
@@ -105,6 +106,13 @@ class Client(
                     type(version).__name__
                 )
             )
+        if utils.version_lt(self._version, constants.MINIMUM_DOCKER_API_VERSION):
+            warnings.warn(
+                'The minimum API version supported is {}, but you are using '
+                'version {}. It is recommended you either upgrade Docker '
+                'Engine or use an older version of docker-py.'.format(
+                    constants.MINIMUM_DOCKER_API_VERSION, self._version)
+            )
 
     @classmethod
     def from_env(cls, **kwargs):

docker/constants.py

@@ -2,6 +2,7 @@ import sys
 from .version import version
 
 DEFAULT_DOCKER_API_VERSION = '1.24'
+MINIMUM_DOCKER_API_VERSION = '1.24'
 DEFAULT_TIMEOUT_SECONDS = 60
 STREAM_HEADER_SIZE_BYTES = 8
 CONTAINER_LIMITS_KEYS = [