Enabling the Go Build Cache on Travis/AppVeyor

The restic project runs the unit and integration tests for each pushed commit and each pull request. We’re using Travis CI for Linux and OS X, and AppVeyor for Windows. We’re very grateful that they provide their services free of charge for Open Source projects!

This is a companion discussion topic for the original entry at https://restic.net/blog/2018-09-02/travis-build-cache