Add -u flag for go get govendor in CONTRIBUTING.md

This to to ensure the reader is not stuck with a stale version
of govendor where `govendor get` may not actually build/install
the final hugo binary.

See kardianos/govendor#330

CONTRIBUTING.md

@@ -97,7 +97,7 @@ You **must use govendor** to fetch and manage Hugo's dependencies.
 ### Fetch the Sources From GitHub
 
 ```
-go get github.com/kardianos/govendor
+go get -u github.com/kardianos/govendor
 govendor get github.com/gohugoio/hugo
 ```