Use packaged version of grpc-tools

--- a/_support/generate-proto-ruby
+++ b/_support/generate-proto-ruby
@@ -27,7 +27,7 @@
     GO_PROTO_LIBS << File.absolute_path(File.join('..', PROTO_INCLUDE))
     includes = GO_PROTO_LIBS.join(":")
 
-    run!(%W[bundle exec grpc_tools_ruby_protoc --proto_path=#{includes} --ruby_out=../#{ruby_lib_spamcheck} --grpc_out=../#{ruby_lib_spamcheck}] + PROTO_FILES)
+    run!(%W[grpc_tools_ruby_protoc --proto_path=#{includes} --ruby_out=../#{ruby_lib_spamcheck} --grpc_out=../#{ruby_lib_spamcheck}] + PROTO_FILES)
   end
 
   write_ruby_requires