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