Lock down rest-client to <= 1.6.7

The 1.6.8 release of rest-client introduced rdoc as a new runtime
dependency. rdoc has ships with fixture data which contains spaces in
the filenames which in turn breaks Omnibus builds on Solaris. Full
details can be found on the original issue:
https://github.com/opscode/chef/pull/1211
Merge pull request #1644 from opscode/schisamo/lock-down-rest-client

Lock down rest-client to <= 1.6.7