tested with mongoid 4 and mongoid 5. Both are fully supported!

bin/nokogiri

@@ -0,0 +1,16 @@
+#!/usr/bin/env ruby
+#
+# This file was generated by Bundler.
+#
+# The application 'nokogiri' is installed as part of a gem, and
+# this file is here to facilitate running it.
+#
+
+require 'pathname'
+ENV['BUNDLE_GEMFILE'] ||= File.expand_path("../../Gemfile",
+  Pathname.new(__FILE__).realpath)
+
+require 'rubygems'
+require 'bundler/setup'
+
+load Gem.bin_path('nokogiri', 'nokogiri')

merge-mongoid.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rspec', '~> 2.14.1' 
   s.add_development_dependency 'rails', '>= 3.2.0'
   s.add_development_dependency 'factory_girl_rails', '~> 3.6.0'
-  s.add_runtime_dependency 'mongoid', "~> 3.1.6"
+  s.add_runtime_dependency 'mongoid', ">= 3.1.6"
 
   s.rubyforge_project = s.name
   s.required_rubygems_version = ">= 1.3.4"