chore(ci): Encrypted notification email

This is to avoid receiving notification from forks. See http://docs.travis-ci.com/user/encryption-keys/

I hope that it will solve the problem of notifications from forks as every repository is supposed to have different encryption key (my fork has different from Elgg/Elgg one).

Side effect with notifications not being sent on failed PRs is a non issue IMHO. We only need info on failed tests after merges as we see test results before merge anyway.