Fix: lock down version of Rack.

Author: dblock

grape-swagger-rails.gemspec

@@ -32,6 +32,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'jquery-rails'
   spec.add_development_dependency 'grape-swagger-ui'
   spec.add_development_dependency 'sprockets'
+  spec.add_development_dependency 'rack', '~> 1.6'
   spec.add_development_dependency 'rack-cors'
   spec.add_development_dependency 'rubocop', '0.33.0'
   spec.add_development_dependency 'mime-types', '< 3.0'