Use 'rubocop' directly

We always used ruby -S rubocop to execute rubocop. We now switch to search for and use a 'rubocop' executable directly. PR made by @jeffbyrnes.

This can be overriden as always by setting the executable key to e.g. here/is/my/rubocop or ['here/is/ruby', '-S', 'rubocop'] in the linter settings.