Revert "pin to an older rainbow due to build issues"

This reverts commit df6a752.

Author: eheydrick

sensu-plugins-postgres.gemspec

@@ -42,7 +42,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'codeclimate-test-reporter', '~> 0.4'
   s.add_development_dependency 'github-markup',             '~> 1.3'
   s.add_development_dependency 'pry',                       '~> 0.10'
-  s.add_development_dependency 'rainbow',                   '2.1.0'
   s.add_development_dependency 'rake',                      '~> 10.0'
   s.add_development_dependency 'redcarpet',                 '~> 3.2'
   s.add_development_dependency 'rspec',                     '~> 3.1'