(PE-5594) Port ring-proxy

Port ring-proxy into the ring-middleware project by modifying it
to use clj-http-client.
(PE-5594) Move functions and change namespace

Change the namespace of the ring-middleware.clj file to
`puppetlabs.ring-middleware.core`. Move utility functions out
of core.clj and into common.clj.
(PE-5594) Change name of test namespace

Change name of test namespace to ring-middleware-test to
core-test.
Remove openjdk6 from .travis.yml

Remove openjdk6 from .travis.yml, as it is not compatible
with Jetty9.
Merge pull request #4 from fpringvaldsen/PE-5594-initial-tests

(PE-5594) Port ring-proxy