Starting to split Ruby and Rails snippets
Changed double quotes to single quotes.

We'll start to change this to make valid Ruby source files according to
Rubocop (https://github.com/bbatsov/rubocop). Changes here came from
https://github.com/honza/vim-snippets/pull/401, thanks for warning us, @mkroman!
Revised the Ruby snippets to conform to Rubocop rules.
Revised the Rails snippets according to Ruby code styles.
Minitest snippets