Add a merge strategy for `VERSION` to automatically use EE version

In https://gitlab.com/gitlab-org/release/docs/blob/master/general/merge-ce-into-ee.md#merging-a-ce-stable-branch-into-its-ee-counterpart, we have this note:

At this point there will be a merge conflict, but likely only on the VERSION file. Resolve it and commit the merge.

See https://medium.com/@porteneuve/how-to-make-git-preserve-specific-files-while-merging-18c92343826b

This will allow RMs to add the following to their ~/.gitconfig:

[merge "ours"]
  driver = true

and completely eliminate the conflict that always happens on VERSION when merging CE into EE.