Fixes #6534 - limit version for rest-client on Ruby 1.8

latest rest-client 1.7.1 dropped support for Ruby 1.8
`<` not `<=` surely?

Actually, I see the same in the hammer-cli-* commits... https://github.com/theforeman/hammer-cli/commit/b944b0d4be772e54bcbdbf518d0a0ff55a5c2249