Switch to Trusty on Travis CI
Switch to Trusty on Travis CI

No need for the PPA repositories anymore.

estan/protoc-gen-doc,DengKe1994/protoc-gen-doc,DengKe1994/protoc-gen-doc,dr-joy/protoc-gen-doc,pseudomuto/protoc-gen-doc,DengKe1994/protoc-gen-doc,estan/protoc-gen-doc,DengKe1994/protoc-gen-doc,dr-joy/protoc-gen-doc,pseudomuto/protoc-gen-doc,estan/protoc-gen-doc