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.
`govendor fetch github.com/hashicorp/terraform/...@v0.11.8`
I figured this was a good time to do it since we just had a release. I verified that everything still compiles and that a few tests are passing- I don't expect anything else to change, but if there are any weird failures that this exposes we'll catch them in CI with plenty of time before the next release.
