TravisCI: Add `hvr-ghc` PPA and install GHC & Cabal
TravisCI: Add `hvr-ghc` PPA and install GHC & Cabal

For the upcoming `check-sdist` CI test suite, we need GHC and Cabal
available on the system.

This commit adjusts `.travis.yml` to add HVR's PPA, install GHC 7.10.3
and Cabal 1.22, and add `~/.cabal` to the cached directories.

NicolasT/reedsolomon,NicolasT/reedsolomon