Use Go Modules and support Go 1.11 building

[josephlr]

This PR makes the necessary changes to get fscrypt properly building with Go 1.11

• Move dependency management from dep to Go's module system.
  • Migrate Gopkg.* files to go.mod and go.sum files
  • Stop vendoring packages needed for developing fscrypt, only vendor those needed to build fscrypt.
    • Makefile now downloads the needed development tools
    • If slow downloads become a problem in Travis, we'll put a package cache in the repo.
    • Instead of using exclude vendor dir from matching ... golang/lint#320 directly, we now use a workaround mentioned in the PR.
• Update all dependencies to their latest versions
  • Update the protoc version and regenerate metadata.pb.go.
  • As protobuf 3.5 now keeps unknown fields, we switch to using proto.Equal to avoid comparison issues with unknown fields.
• Move Travis to Go 1.11
  • This requires rerunning gofmt because of golang/go@542ea5a