Fix stackdriver tests (census-instrumentation#921)

Fixes census-instrumentation#905 by pinning rsa for unsupported python versions.

Author: aabmass

contrib/opencensus-ext-stackdriver/setup.py

@@ -41,6 +41,7 @@
     install_requires=[
         'google-cloud-monitoring >= 0.30.0, < 1.0.0',
         'google-cloud-trace >= 0.20.0, < 1.0.0',
+        'rsa <= 4.0; python_version<="3.4"',
         'opencensus >= 0.8.dev0, < 1.0.0',
     ],
     extras_require={},