Add a spec:all task to run specs against both ORMs

.travis.yml

@@ -0,0 +1,4 @@
+language: ruby
+services:
+  - mongodb
+script: bundle exec rake spec:all

Rakefile

@@ -3,3 +3,14 @@ require 'bundler/gem_tasks'
 require 'rspec/core/rake_task'
 RSpec::Core::RakeTask.new(:spec)
 task :default => :spec
+
+desc 'Run the test suite for all supported ORMs.'
+namespace :spec do
+  task :all do
+    %w[active_record mongoid].each do |orm|
+      ENV["ORM"] = orm
+      Rake::Task["spec"].reenable
+      Rake::Task["spec"].invoke
+    end
+  end
+end