Fixing up the disable gRPC env. var. name.

Was accidentally maintained as GAX instead of GRPC
in a rebase.

Author: dhermes

google/cloud/environment_vars.py

@@ -36,7 +36,7 @@
 CREDENTIALS = 'GOOGLE_APPLICATION_CREDENTIALS'
 """Environment variable defining location of Google credentials."""
 
-DISABLE_GRPC = 'GOOGLE_CLOUD_DISABLE_GAX'
+DISABLE_GRPC = 'GOOGLE_CLOUD_DISABLE_GRPC'
 """Environment variable acting as flag to disable gRPC.
 
 To be used for APIs where both an HTTP and gRPC implementation