use .dup to not send frozen strings into method that modifies args

`RUBY_VERSION` is a 'frozen' string, and apparently older versions of `Gem::Version` used `strip!` in its internals, so doing `Gem::Version.new(RUBY_VERSION)` implodes: https://github.com/rubygems/rubygems/commit/da4362a6644ca5a75c210677ac500bccfe75f529

but it should be safe to preventatively send in a copy of the string using `.dup`. so let's try that.