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
setting proper semver constraint on ffi-yajl

ffi-yajl 1.0.0 has been relesed
Fix cookbook_site_share tests
Hack around system differences for travis test
Include the shell_out mixin in cookbook_site_share