Loosen mongoid version requirement

Author: elia

activeadmin-mongoid.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |gem|
   gem.version       = ActiveAdmin::Mongoid::VERSION
   gem.license       = 'MIT'
 
-  gem.add_runtime_dependency 'mongoid',         '~> 4.0'
+  gem.add_runtime_dependency 'mongoid',         ['>= 4.0', '< 5']
   gem.add_runtime_dependency 'rails',           '~> 4.0'
   gem.add_runtime_dependency 'activeadmin',     '~> 0.6.1'