Update RubyLinter

[kaste]

We have a really simple to read RubyLinter base class in production since 26 Aug 2019 for rubocop
https://github.com/SublimeLinter/SublimeLinter-rubocop/blob/master/linter.py#L5-L22 The simplified version is (more or less) even in 3.0.0 (27 May 2019) where we didn't even used the base class from SL core.

The plan is to copy this as the new base here in SL core. The old base class is hard to comprehend, afaik doesn't even fully support our executable setting.

When doing this all "ruby" linters (plugins) must be checked if they have a sane cmd. Some of them for example define ruby -S foolint as their cmd which is really against our standards, and in that case executable is for sure not supported because it would only exchange the ruby part here. Actually, executable supports settings like ["ruby", "-S", "foolint"] and with it enables users to customize everything.

All plugin cmds must have the simple form foolint -a -r -g -s (either as string or tuple/list), so that we can do a simple which(foolint) as well as any other more fancy expansion.

[jeffbyrnes]

I ran into issues w/ RubyLinter just the other day; I might try & spend some cycles on this soon.