cosmetic fixes

### Problem 
The active_shipping project has a few stylistic inconsistencies. Let's clean this up since we'll be owning it. Also, this gives me a chance to learn our coding style. Lemme know if I misinterpreted https://vault.shopify.com/Ruby pls. @RichardBlair mentioned to use IRB style for things not in the style guide.

### Solution
Apply non-functional changes to the source.

Protip: https://github.com/Shopify/active_shipping/pull/197/files?w=1 hides boring whitespace changes.
pls2review @Shopify/shipping @costford 
Why the underscore change?