Bump protobuf compiler to v3.6.1

Full list of changes: https://github.com/protocolbuffers/protobuf/blob/master/CHANGES.txt

Some changes relevant to us:

• Added ruby_package file option to control the module of generated class.
• Fixed a bug introduced in 3.5.0 and protoc in Windows now accepts non-ascii characters in paths again.
• reserve keyword are now supported in enums
• Extension ranges now accept options and are customizable.
• "reserve" keyword now supports “max” in field number ranges, e.g. reserve 1000 to max;
• Added protoc version number to protoc plugin protocol. It can be used by protoc plugin to detect which version of protoc is used with the plugin and mitigate known problems in certain version of protoc.

This mostly updates the compiler to match the update to the protobuf gem in both GitLab CE and Gitaly.