Added ruby-head to travis allowed-failures

This is probably a contraversial idea. ruby-head can be fairly unstable
but then json is a core gem. At least if it's an allowed-failure then
builds will always be run against ruby-head, but pull requests (and
master) won't report build failures if something is broken in ruby-head

Author: iainbeeston

.travis.yml

@@ -18,4 +18,5 @@ matrix:
   allow_failures:
     - rvm: rbx-18mode
     - rvm: rbx-19mode
+    - rvm: ruby-head
 script: "bundle exec rake"