Move MR Builds tab next to Commits

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/4132

@dzaporozhets We've got two people who feel strongly about this now, @sytses filed an issue for it :)

![Screen_Shot_2015-12-16_at_22.19.35](/uploads/37e705610ba54e386928d7972398c247/Screen_Shot_2015-12-16_at_22.19.35.png)


See merge request !2128