Enforce docker>=4.2.1 on pip install

Signed-off-by: Ulysses Souza <[email protected]>

setup.py

@@ -37,7 +37,7 @@ install_requires = [
     'texttable >= 0.9.0, < 2',
     'websocket-client >= 0.32.0, < 1',
     'distro >= 1.5.0, < 2',
-    'docker[ssh] >= 3.7.0, < 5',
+    'docker[ssh] >= 4.2.1, < 5',
     'dockerpty >= 0.4.1, < 1',
     'six >= 1.3.0, < 2',
     'jsonschema >= 2.5.1, < 4',