configure Stem's Travis CI to email atagar and the committer when there are failures
I need to:
- check stem's Travis CI github.com/torproject/stem site config to make sure we haven't disabled emails
- configure emails to the committer and atagar on github.com/torproject/stem, or if that's not possible, in .travis.yml
- the difference is that the site will always be running stem master, but other people's repositories can have old stem versions