Fix for Rails syntax.

There is now little point in running this checker rather than the plain
ruby one.
Make shell escaping less produce fewer useless quotes.

Function inspired by tpope's vim-dispatch.
Refactor FindInParent from haxe.vim to be a util function
Fix Elixir `mix` detection.

Syntax checker now looks up thorugh its parents to find if it is part
of a mix project.
Mix needs the cwd to be set to the mix file.
Cleanup.
Merge branch 'master' into gcc_refactor