Update golint import path

The golint package updated its import path in commit c363707d6884 [1],
from github.com/golang/lint to golang.org/x/lint. The github repo is now
merely a mirror, as it seems.

Follow this and update github.com/golang/lint/golint to the new import
path golang.org/x/lint/golint.

[1] golang/lint@c363707d6884

.travis.yml

@@ -8,7 +8,7 @@ sudo: false
 install:
   - go get github.com/go-stack/stack
   - go get github.com/sirupsen/logrus
-  - go get github.com/golang/lint/golint
+  - go get golang.org/x/lint/golint
   - go get github.com/kr/pretty
 
 script: