Relax dependency on a specific version of Mongoid, compatibility with Mongoid 4.x.

This is a trivial gemspec change that makes this gem compatible with Mongoid 4. No code changes were necessary. Since that hasn't shipped you would need to do this in Gemfile to test:

``` ruby
gem "mongoid", ">= 3.0", github: "mongoid/mongoid"
gem "mongoid_slug", github: "dblock/mongoid-slug", branch: "mongoid-4x"
```