Switch out to a different PPA for some dependencies.

That new PPA seems dedicated to providing backports for older versions of
Ubuntu, rather than supporting just what this person needs, as the one we
were using before this commit. So I hope it's more stable...
Disable Travis notification emails.

This doesn't work as well as I hoped, and tends to have forks spamming us.
Merge branch 'master' into super_duper
Add a PPA for libstdc++-4.8-dev.