Switch to using Go modules.
This migrates our vendor.json to use Go 1.11's modules system, and
replaces the vendor folder with the output of go mod vendor.
The vendored code should remain basically the same; I believe some
tree shaking of packages and support scripts/licenses/READMEs/etc.
happened.
This also fixes Travis and our Makefile to no longer use govendor.
vendoring the beta API into TPG too makes it so we don't have to have a ton of conditionals in the composer resources. We can just use the beta API all the time, but only send beta fields in the beta provider (this is what we're doing already for basically everything else)
