Fix cleanup_file_history command after adjustments to Python3
Remove checks and migration for the "old settings file"

This old version has never been in Package Control and probably is so old
that no one is using it anymore. And even so the damage wouldn't be severe.
Also use `cls` instead of `FileHistory` when applicable.
Update README

ST3 compatible now and use relative links as github now supports them.
Fix exception when there is no view in the current group