If you're using golang/dep, and for an unexplained reason running dep ensure
errors:
unable to update checked out version: : command failed: [git checkout 019ae5ada31de202164b118aee88ee2d14075c31]: exit status 1
You have to remove the dep from your Gopkg.lock file. This way Gopkg won't look for the missing ref.
And then dep ensure
again.