.travis: Update golint URI to use x/lint
.travis: Update golint URI to use x/lint

Catching up with golang/lint@ead987a6 (README.md: update "go get" link
to use x/lint repo, 2018-02-21, haruyama/golintx#20) and avoiding [1]:

  $ go get github.com/golang/lint/golint
  package github.com/golang/lint/golint: code in directory
  /home/travis/gopath/src/github.com/golang/lint/golint expects import
  "golang.org/x/lint/golint"

[1]: https://travis-ci.org/dmacvicar/terraform-provider-libvirt/builds/440782401#L517

dmacvicar/terraform-provider-libvirt,dmacvicar/terraform-provider-libvirt,dmacvicar/terraform-provider-libvirt