Downgrade bundler requirement for CI.

multibinder.gemspec

@@ -17,6 +17,6 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]
 
-  spec.add_development_dependency "bundler", "~> 1.6"
+  spec.add_development_dependency "bundler", "~> 1.5"
   spec.add_development_dependency "rake", "~> 10.0"
 end