Reworked `.travis.yml` for GHC 7.6 builds.

I used [hvr's GHC versions PPA](http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/20634). This is nice because a lot of build failures lately have been 7.4-specific. It's a little less configurable than the old one, but it wouldn't be hard to reimplement the benchmarking stuff if someone really wanted to keep it -- on the other hand, it seemed silly for only certain configurations to have `--show-details=always`.

(This branch does build on 7.6.3 -- the only test failure for 7.4.2 is a [weird GHC panic](https://travis-ci.org/ekmett/lens/jobs/12863678#L1552) during `doctests`.)