Update coveralls from 0.8.9 to 0.8.13 and simplecov from 0.10.0 to 0.11.2

This removes a few dependencies! It was also rude to be using coveralls
0.8.9, considering 0.8.12 introduced support for GitLab CI :) Also
paves the way for updating mime-types to 3.0.

Coveralls Changelog:
https://github.com/lemurheavy/coveralls-ruby/releases

Simplecov Changelog:
https://github.com/colszowka/simplecov/blob/master/CHANGELOG.md