simplecov 0.18.0.beta2 on Ruby Gems

Enhancements

only turn on the requested coverage criteria (when activating branch coverage before SimpleCov would also instruct Ruby to take Method coverage)
Change how branch coverage is displayed, now it's branch_type: hit_count which should be more self explanatory. See #830 for an example and feel free to give feedback!
Allow early running exit tasks and avoid the at_exit hook through the SimpleCov.run_exit_tasks! method. (thanks @macumber)
Allow manual collation of result sets through the SimpleCov.collate entrypoint. See the README for more details (thanks @ticky)
Within case, even if there is no else branch declared show missing coverage for it (aka no branch of it). See #825
Stop symbolizing all keys when loading cache (should lead to be faster and consume less memory)
Cache whether we can use/are using branch coverage (should be slightly faster)

Fix a crash that happened when an old version of our internal cache file .resultset.json was still present