[AIRFLOW-412] Fix lxml dependency

Dear Airflow Maintainers,

Please accept this PR that addresses the following issues:
- https://issues.apache.org/jira/browse/AIRFLOW-412

Testing Done:
-None

Closes #1722 from normster/lxml

setup.py

@@ -202,6 +202,7 @@ def do_setup():
             'tabulate>=0.7.5, <0.8.0',
             'thrift>=0.9.2, <0.10',
             'zope.deprecation>=4.0, <5.0',
+	    'lxml==3.6.0',
         ],
         extras_require={
             'all': devel_all,