Added the rainbow dependency to the readme

In the README.md, added the rainbow dependency along with the other
dependencies of the project.
Merge pull request #255 from gilles-leblanc/add_rainbow_dep_to_readme

Adds the rainbow dependency to the readme
Force ruby2ruby 2.0.8.
Bump version to 1.3.7.
Avoid duplicate method calls

In support of #231, here's a little fix to take
a couple of things off that warning count!
Merge pull request #256 from andyw8/avoid_duplicate_method_calls

Avoid duplicate method calls
Specify required Rainbow version as >= 1.99
Merge pull request #258 from troessner/rainbow_dep

Specify required Rainbow version as >= 1.99