Add runtime dependency for rainbow gem

Author: Peter Benjamin

source/restacker.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |s|
   s.files   = `git ls-files -z`.split("\x0")
   s.require_paths = ["lib"]
   s.add_runtime_dependency 'aws-sdk', '~> 2'
-  s.add_runtime_dependency 'rainbow'
+  s.add_runtime_dependency 'rainbow', '~> 2'
   s.homepage = 'https://github.com/devsecops/restacker'
   s.license = 'Apache License Version 2.0'
 end