[AIRFLOW-3193] Pin docker requirement version (#4130)

Since the change of AIRFLOW-3203, the dockerpy client has upgraded
actually. Therefore, the requirements config should upgrade also to
avoid potentially misleading.

setup.py

@@ -174,7 +174,7 @@ doc = [
     'sphinx-rtd-theme>=0.1.6',
     'Sphinx-PyPI-upload>=0.2.1'
 ]
-docker = ['docker>=2.0.0']
+docker = ['docker>=3.0.0']
 druid = ['pydruid>=0.4.1']
 elasticsearch = [
     'elasticsearch>=5.0.0,<6.0.0',