Use require instead of require_relative for improved Ruby 1.8 support

I tried adding `emoji-rspec` to [one of my projects](https://github.com/sferik/t) but the [specs are failing](https://travis-ci.org/sferik/t/jobs/4803975) on Ruby 1.8.7 due to your use of `require_relative`. As far as I can tell, there's not reason not to simply use `require` here.