Build on Travis with hvr's GHC versions PPA.

Preserve other environment variables for travis.

I didn't really think about what would happen with $mode and $script.
>.>

Install dependencies for travis in the `install` step.

Rewrite the .travis.yml almost completely.

This .travis.yml allows us to specify versions more nicely and also fall
back to travis' GHC if it's the same version as the one we want.

I'd be OK with adding benchmarking options back, but $script seems like
a weird way to do it.

Install test dependencies for travis with cabal-apt-install.

This should make things a little quicker.
Ignore 7.4.2 travis failures (re: doctests).

Ideally this will be a temporary measure.