Prepare for 0.15.2 release (#491)

Also, remove overview.rb from golden test manifest

Dockerfile

@@ -1,9 +1,9 @@
 FROM ubuntu:16.04
 
 # Release parameters
-ENV GOOGLEAPIS_HASH a31e5f072dbf18a493efabbca9e39e200ed1a78e
-ENV GAPIC_GENERATOR_HASH b1d46dec256e433e07a11992a41b9e3cf0df54ea
-ENV ARTMAN_VERSION 0.15.1
+ENV GOOGLEAPIS_HASH bdd31f00f7c52fc8075e2bf476a5253f5c60c2ba
+ENV GAPIC_GENERATOR_HASH cc12bdf2be42d53266748320f91df53392131a18
+ENV ARTMAN_VERSION 0.15.2
 
 ENV DEBIAN_FRONTEND noninteractive
 

setup.py

@@ -23,7 +23,7 @@
 
 # The following line is parsed by CI scripts (see .circleci/config.yml)
 # and by release.py. Please keep the format.
-current_version = '0.15.1'
+current_version = '0.15.2'
 
 cur_dir = os.path.realpath(os.path.dirname(__file__))
 with io.open('%s/requirements.txt' % cur_dir) as requirements_file:

test/golden/library_example.golden

@@ -153,7 +153,6 @@
 /ruby/google-cloud-ruby/google-cloud-library_example/lib/google/cloud/example/library/v1/credentials.rb
 /ruby/google-cloud-ruby/google-cloud-library_example/lib/google/cloud/example/library/v1/doc/google/example/library/v1/library.rb
 /ruby/google-cloud-ruby/google-cloud-library_example/lib/google/cloud/example/library/v1/doc/google/protobuf/empty.rb
-/ruby/google-cloud-ruby/google-cloud-library_example/lib/google/cloud/example/library/v1/doc/overview.rb
 /ruby/google-cloud-ruby/google-cloud-library_example/lib/google/cloud/example/library/v1/library_service_client.rb
 /ruby/google-cloud-ruby/google-cloud-library_example/lib/google/cloud/example/library/v1/library_service_client_config.json
 /ruby/google-cloud-ruby/google-cloud-library_example/lib/google/example/library/v1/library_pb.rb