Use require instead of require_relative for improved Ruby 1.8 support

I tried [adding `emoji-rspec`](https://github.com/sferik/t/commit/14394158e9226b56252ef35493101ed4d60c9c06) to [one of my projects](https://github.com/sferik/t) but the [specs are failing on Ruby 1.8.7](https://travis-ci.org/sferik/t/jobs/4803975) due to your use of `require_relative`.

As far as I can tell, there's not reason not to simply use `require` here instead.