bump ruby google-protobuf dependency

grpc.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |s|
   s.require_paths = %w( src/ruby/lib src/ruby/bin src/ruby/pb )
   s.platform      = Gem::Platform::RUBY
 
-  s.add_dependency 'google-protobuf', '~> 3.1'
+  s.add_dependency 'google-protobuf', '~> 3.7'
   s.add_dependency 'googleapis-common-protos-types', '~> 1.0.0'
 
   s.add_development_dependency 'bundler',            '~> 1.9'

templates/grpc.gemspec.template

@@ -31,7 +31,7 @@
     s.require_paths = %w( src/ruby/lib src/ruby/bin src/ruby/pb )
     s.platform      = Gem::Platform::RUBY
 
-    s.add_dependency 'google-protobuf', '~> 3.1'
+    s.add_dependency 'google-protobuf', '~> 3.7'
     s.add_dependency 'googleapis-common-protos-types', '~> 1.0.0'
 
     s.add_development_dependency 'bundler',            '~> 1.9'