Lock rest-client to ~> 1.6.0 on Ruby 1.8.7
Only set the included flag when an equalizer instance is included.

This spec was failing on RSpec 2.99 and 3.0 due to the following:

- It redefines `Module#included` globally, meaning that a module
  include done by anything would execute our `define_method` block.
- RSpec uses autoload to lazily load matchers on first use.
- The implementation of the `change` matcher includes a module.
- Thus, when `change` was called (but before `matches?` is called
  on it), the change definition was loaded and it included a
  module, changing the `included` flag prematurely and causing
  it to fail.

See rspec/rspec-core#1617 for a longer discussion.