Fix RuboCop offenses

If you want to use single-quoted string when you don't need string interpolations or special symbols, remove lines from `.rubocop.yml`:
``` yml
Style/StringLiterals:
  EnforcedStyles: double_quotes
```
then run `rubocop --auto-correct`. Since the default `EnforcedStyles` of `Style/StringLiterals` is `single_quotes`, removing those lines will work. You can see the default configuration file [default.yml](https://github.com/bbatsov/rubocop/blob/master/config/default.yml) of RuboCop on GitHub.