fix for installation error

based on suggestion: googleapis/google-cloud-python#5703

Author: pgm

setup.py

@@ -22,6 +22,7 @@
       install_requires=[
           'termcolor==1.1.0',
           'attrs==17.2.0',
+          'googleapis-common-protos==1.5.3',
           'google-cloud-datastore==1.4.0',
           'google-cloud-storage==1.6.0',
           'grpcio-tools==1.13.0',