core: clean up tests for synced folder built-ins
core: better synced folder plugin docs
core: update rspec, fix deprecations
core: only set I18n setting on newer versions

/cc @fgrehm - Looks like only the later versions have this.
providers/virtualbox: clean up synced folders on destroy
kernel/v2: fix validation on checksum types
core: fix exception when checksum type is nil

Thanks @johnbellone
core: note Ruby and RubyGems version in logs
commands/plugin: ignore default gems when pruning

For modern RubyGems compatibility