fix RuboCop lint warnings

Don't mind me... just having a little OCD moment.

![shaking](http://media.giphy.com/media/zMaDW3QwlQ2bu/giphy.gif)

```bash
$ rubocop -R -l
```

https://github.com/bbatsov/rubocop
I'm going to say that it probably isn't working :wink: 

https://github.com/18F/C2/blob/47cca0f9fc44cb66c6ac7177865065dbb87946a8/app/mailers/communicart_mailer.rb#L68-L70