`razor-client` cannot install on Ruby < 1.9.2

razor-client had a versionless dependency on the rest-client gem. Previously,
rest-client 1.6.8 would work with Ruby >= 0. Today, rest-client issued a new
release (1.7.0) that only works with Ruby >= 1.9.2.

The fix, since we need to support older versions of Ruby, is to pin the
rest-client gem to versions prior to 1.7.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-342