Tighten the dependency on rainbow gem

Rexec does not really work with any version of rainbow gem. It
only currently works with rainbow version "~> 2.0.0" because lesser
versions do not have string extension to begin with.

This causes problems for upstream users of this gem or any gem
that uses rexec.
Merge pull request #3 from code-mancers/master

Tighten the dependency on rainbow gem