Tell Travis CI to notify the PDC WebHook

Without this patch the so-called "puppet development community" hook
service isn't being notified when Travis CI accepts jobs.  This is a
problem because we want the ability for Travis Bot to comment on pull
requests as a result of CI build results.  For example, if the build fails,
then Gepetto Bot could make some helpful suggestions on how to re-run the
build by amending a commit and force-pushing the branch.

This patch uses the notifications section of the travis.yml configuration
file, as documented at:
http://about.travis-ci.org/docs/user/notifications/#Webhook-notification