Explicitly set source to https://rubygems.org in Gemfile

Otherwise `bundle install` command reports:

> The source :rubygems is deprecated because HTTP requests are insecure.
> Please change your source to 'https://rubygems.org' if possible, or
'http://rubygems.org' if not.
Update gemspec to accept Mongoid ~> 3.1 versions too.

All specs still pass.
Add mongodb service to Travis CI config

Without mongodb all specs FAIL.
Do not use Rakefile for run specs on Travis CI
Disable Ruby 1.9.2 check due to Mongoid requirement

Mongoid requires MRI version 1.9.3+ or JRuby 1.6.0+ running in 1.9 mode.
Please see: http://mongoid.org/en/mongoid/docs/tips.html#ruby for
details.
Merge pull request #14 from alsemyonov/master

Make mongoid-ancestry compatible with Mongoid 3.1