Remove Travis notifications: section.

It had three problems:

 - it always notifies the Cloud Haskell maintainer email address (and
   the #haskell-distributed IRC channel), even for pushes done in
   a fork (public or private) that the Cloud Haskell maintainer has no
   control over.

 - it inhibited notifying the actual committers and authors the way the
   default does.
Merge pull request #160 from tweag/travis-notifications

Remove Travis 'notifications:' section.