Install GHC 7.8.2 instead of 7.6.3

This is a little awkward in that experimental pulls 7.8.1 right now, but it should provide 7.8.2 by the time this pull request gets merged. `-t experimental` should get changed to `-t unstable` once the 7.8 series makes it there.

Also, are `/etc/apt/sources` configured for experimental? If not, it should be as simple as adding `experimental` after `main` in the `sources` list. https://wiki.debian.org/DebianExperimental