[RDY] Improve neovim.rb

1. Fix the conflict between `md5sha1sum` and `polarssl`. Both of them provide the same CLT.
2. Add `=> :build` to reduce warning in `brew audit`.
3. Modernize, replace single quotes to double quotes. https://github.com/styleguide/ruby
As @xu-cheng said, it seems to be Homebrew style: https://github.com/Homebrew/homebrew/pull/30449 (it's in one of the outdated comments) By Mike McQuaid. I noticed this recently because it concerns something I wrote and someone tried to submit a homebrew formula.

I think the best compromise is to split the PR into a functionality change commit and a style change commit. Can you do that @xu-cheng? It doesn't matter which commit you do first.