Switch to Ruby's native JSON parser

Ruby's JSON parser is ~20x faster, because it's written in C.

```
Calculating -------------------------------------
                ruby       204 i/100ms
                rack         6 i/100ms
-------------------------------------------------
                ruby     2071.8 (±5.1%) i/s -      10404 in   5.034635s
                rack       68.6 (±4.4%) i/s -        342 in   5.001516s
```