Changed `Commit` to `Build` header on builds table

## What does this MR do?

As [discussed here](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6322#note_15511777) the pipeline table `Commit` headers should be `Pipeline`. This also should apply to build table headers.

## Are there points in the code the reviewer needs to double check?

/cc @tauriedavis 

## Why was this MR needed?

UI consistency

## Screenshots (if relevant)

![Screen_Shot_2016-09-18_at_16.39.16](/uploads/61e64fb3152ac3ec81bec35c5ba0e312/Screen_Shot_2016-09-18_at_16.39.16.png)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !6402