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

/cc @opscode/release-engineers @opscode/client-engineers