Add CI tests to check that all proto files are generated with the same version

[tsehori]

We want to deal with the possible inconsistency between versions of protocol buffer generation using protoc-gen-go, as developers that use different versions may hit issues when building and/or testing their Fleetspeak deployments.

To overcome this, we want to add tests to Travis CI that regenerate the proto files using a fixed version of protoc-gen-go and diff them with the generated files in submitted pull requests. This can be done by using git diff that should be accessible through Travis CI.