Make Rack::Test a development dependency

sinatra-contrib/lib/sinatra/test_helpers.rb

@@ -1,6 +1,10 @@
 require 'sinatra/base'
-require 'rack/test'
 require 'rack'
+begin
+  require 'rack/test'
+rescue LoadError
+  abort 'Add rack-test to your Gemfile to use this module!'
+end
 require 'forwardable'
 
 module Sinatra

sinatra-contrib/sinatra-contrib.gemspec

@@ -220,7 +220,6 @@ Gem::Specification.new do |s|
   s.add_dependency "mustermann",  "1.0.0.beta2"
   s.add_dependency "backports", ">= 2.0"
   s.add_dependency "tilt",      ">= 1.3", "< 3"
-  s.add_dependency "rack-test"
   s.add_dependency "rack-protection", Sinatra::VERSION
   s.add_dependency "multi_json"
 
@@ -242,4 +241,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency "wikicloth"
   s.add_development_dependency "markaby"
   s.add_development_dependency "rake", "< 11"
+  s.add_development_dependency "rack-test"
 end