.travis.yml: Use golint's preferred import path.

Previously, tests failed with:

$ 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"

.travis.yml

@@ -5,7 +5,7 @@ before_install:
   - go get github.com/axw/gocov/gocov
   - go get github.com/mattn/goveralls
   - go get golang.org/x/tools/cmd/cover
-  - go get github.com/golang/lint/golint
+  - go get golang.org/x/lint/golint
 before_script:
   - go get -d ./...
 script: