Enable OS X builds on Travis CI.

Travis-CI recently began supporting multi-OS builds:

    http://blog.travis-ci.com/2014-05-13-multi-os-feature-available/

Since we don't have a OS X 64bit build slave currently, let's see if
Travis can help out for now.
No apt-get on osx.
Try out Pers suggestion in #1473.
Merge github.com:lkarsten/Varnish-Cache into paced_socket
Clean up.