Fix incompatible httplib2 versions (#564)

Our build is broken due to this error message:

pkg_resources.ContextualVersionConflict: (httplib2 0.17.0 (/home/travis/build/googlegenomics/gcp-variant-transforms/.eggs/httplib2-0.17.0-py2.7.egg), Requirement.parse('httplib2<=0.12.0,>=0.8'), set(['apache-beam']))
Coverage.py warning: No data was collected. (no-data-collected)

For more information please refer to issue #71

setup.py

@@ -35,7 +35,7 @@
     # Note that adding 'google-api-python-client>=1.6' causes some dependency
     # mismatch issues. This is fatal if using 'setup.py install', but works on
     # 'pip install .' as it ignores conflicting versions. See Issue #71.
-    'google-api-python-client>=1.6',
+    'google-api-python-client>=1.6,<1.7.12',
     'intervaltree>=2.1.0,<2.2.0',
     'mmh3<2.6',
     # Refer to issue #528