Don't take pull requests into account for branches list

When we build a branches list, we should not take pull requests into
account, because they're not really on the given branch - they're just
targetted at it.
Merge pull request #329 from travis-ci/ps-get-only-pushes-for-branch-list

Don't take pull requests into account for branches list
Fix branches ordering

Previous way of ordering was ordering by name first, than by
finished_at. It resulted in getting 25 first branches by name and then
ordering it by finished_at DESC.

This commit changes the situation to get all newest builds per branch,
then order it by finished_at DESC and then limit them to 50.
No need to order inner query when getting branches
Merge pull request #330 from travis-ci/ps-proper-branches-order

Fix branches ordering
Revert "No need to order inner query when getting branches"

This reverts commit 58a11d21cd15173577040129874ce50f0b72a1d3.
Merge branch 'master' into hh-add-links-to-jobs