Fix adding/replacing a grammar

[lildude]

Description

Whilst looking into the problems with #4088, I discovered adding a new grammar broke with the merging of #3982 as follows:

$ script/add-grammar https://github.com/munificent/wren-sublime
Registering new submodule: vendor/grammars/wren-sublime
$ git submodule add -f https://github.com/munificent/wren-sublime vendor/grammars/wren-sublime
$ script/grammar-compiler add vendor/grammars/wren-sublime
Confirming license
$ script/licensed --module vendor/grammars/wren-sublime
  > Caching licenes for linguist:
  >   grammar dependencies:
  > /Users/lildude/github/linguist/vendor/gems/ruby/2.4.0/gems/licensed-1.0.0/lib/licensed/dependency.rb:19:in `initialize': Dependency path vendor/grammars/wren-sublime must be absolute (RuntimeError)
  > 	from script/licensed:26:in `new'
  > 	from script/licensed:26:in `block in dependencies'
  > 	from script/licensed:24:in `map'
  > 	from script/licensed:24:in `dependencies'
  > 	from /Users/lildude/github/linguist/vendor/gems/ruby/2.4.0/gems/licensed-1.0.0/lib/licensed/command/cache.rb:27:in `block (3 levels) in run'
  > 	from /Users/lildude/github/linguist/vendor/gems/ruby/2.4.0/gems/licensed-1.0.0/lib/licensed/command/cache.rb:20:in `map'
  > 	from /Users/lildude/github/linguist/vendor/gems/ruby/2.4.0/gems/licensed-1.0.0/lib/licensed/command/cache.rb:20:in `block (2 levels) in run'
  > 	from /Users/lildude/github/linguist/vendor/gems/ruby/2.4.0/gems/licensed-1.0.0/lib/licensed/command/cache.rb:17:in `chdir'
  > 	from /Users/lildude/github/linguist/vendor/gems/ruby/2.4.0/gems/licensed-1.0.0/lib/licensed/command/cache.rb:17:in `block in run'
  > 	from /Users/lildude/github/linguist/vendor/gems/ruby/2.4.0/gems/licensed-1.0.0/lib/licensed/command/cache.rb:12:in `each'
  > 	from /Users/lildude/github/linguist/vendor/gems/ruby/2.4.0/gems/licensed-1.0.0/lib/licensed/command/cache.rb:12:in `flat_map'
  > 	from /Users/lildude/github/linguist/vendor/gems/ruby/2.4.0/gems/licensed-1.0.0/lib/licensed/command/cache.rb:12:in `run'
  > 	from script/licensed:53:in `<main>'
  > caching vendor/grammars/wren-sublime
$

This was because we missed a spot where we needed to pass in a full repo path to licensed.

Whilst I'm at it, I've also put a quick guard to ensure Docker is installed and running so users get a nicer failure experience:

$ script/add-grammar https://github.com/munificent/wren-sublime
Detecting docker
$ docker ps
  > Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
$

I've also updated the CONTRIBUTING.md to point out Docker is required for adding or replacing grammars.

Checklist:

• I am adding new or changing current functionality
  • I have added or updated the tests for the new or changed functionality.

[Alhadis]

Tested locally. It works. 👍

In the sense of not working in a cleaner fashion.

[Alhadis]

@lildude I've removed my irrelevant ramblings and my attempt at "refactoring" I got sidetracked by while learning Ruby and was too sleep-deprived at the time to realise what an absurd mess I was making.

Crap reverted and superseded by a fix that resembles something not written by a rambling lunatic.

Testing locally right now. Pushing in advance out of embarrassment.