Add grpcio-gcp dependency for API Core and Cloud Spanner (#5904)

Author: WeiranFang

setup.py

@@ -22,7 +22,7 @@
 
 name = 'google-api-core'
 description = 'Google API client core library'
-version = '1.3.0'
+version = '1.4.0'
 # Should be one of:
 # 'Development Status :: 3 - Alpha'
 # 'Development Status :: 4 - Beta'
@@ -39,7 +39,8 @@
     'futures>=3.2.0;python_version<"3.2"'
 ]
 extras = {
-    'grpc': 'grpcio>=1.8.2'
+    'grpc': 'grpcio>=1.8.2',
+    'grpcio-gcp': 'grpcio-gcp>=0.2.2'
 }