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
Merge pull request #221 from iainbeeston/allow_ruby_head_to_fail

Allow ruby-head builds to fail