Removed all warnings from the Python/C++ build.

Also made the Travis build ensure that no warnings
are present.

These builds were previously spewing many warnings,
which was hiding warnings for important things like
accidentally using C++11-only features.

Change-Id: I56caeee9db48bc78756a3e8d7c14874630627037
diff --git a/python/tox.ini b/python/tox.ini
index a6352ef..7deb3ba 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -13,7 +13,7 @@
 commands =
     python setup.py -q build_py
     python: python setup.py -q build
-    cpp: python setup.py -q build --cpp_implementation
+    cpp: python setup.py -q build --cpp_implementation --warnings_as_errors
     python: python setup.py -q test -q
     cpp: python setup.py -q test -q --cpp_implementation
 deps =