Fixed hg's update method so that it pulls from remote before updates

[hegyigee]

Hi there! I got here from glide. We use bitbucket hg repos for our go projects.

When a project dependency gets updated in the remote repo we then update the version glide.yaml and run glide up but it throws an error saying unknown version.

I checked the code and it turned out that only in case of hg the Update() method executes only hg update which does not fetch changes from the remote repo, it only updates the version to the latest known locally.

[mattfarina]

@hegyigee thanks for the fix!