Rubocop - string literals

Based off of #5, so merge that first. I realize this might be controversial, so I put it in another pull request.

This enables the Rubocop rule that string literals should be single quotes unless they need to be double quotes (e.g. they contain a single quote or do interpolation).

My rationalization for this is:

- it enforces consistency
- `rubocop -a` will fix any of these violations for you