[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
these cosmetic changes distract from the real change. try to avoid this.