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