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
(maint) Update puppet version in Travis configuration

Without this patch we'll test against all Puppet 3.0 versions, but not
4.  This is a problem because the travis configuration should be
relatively future proof to minimize surprises.

This patch addresses the problem by using >= instead of ~> in the
dependency specification.  The patch also combines two different
notification sections into a single entry.
Merge pull request #123 from jeffmccune/feature/master/travis_webhook

Tell Travis CI to notify the PDC WebHook