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.
Parallel to https://github.com/terraform-providers/terraform-provider-azurerm/pull/2116
We need to run `gofmt -s` to switch over to the `gofmt` rule, and MM is going to complicate that. Once `1.20.0` is done, I'll run that command on flat MM files & add a `gofmt -s` step to MM generation (goimports does gofmt but doesn't do the s flag for silly reasons).
I'll duplicate this over to tpgb once this PR is merged.
