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.