Remove ceiling on required protobuf version

This is possible now that https://github.com/google/protobuf/issues/855
is fixed (or at least appears to be fixed).
diff --git a/src/python/grpcio/setup.py b/src/python/grpcio/setup.py
index b8a98c3..366ebe3 100644
--- a/src/python/grpcio/setup.py
+++ b/src/python/grpcio/setup.py
@@ -146,7 +146,7 @@
 
 TESTS_REQUIRE = (
     'oauth2client>=1.4.7',
-    'protobuf==3.0.0a3',
+    'protobuf>=3.0.0a3',
     'coverage>=4.0',
 ) + INSTALL_REQUIRES