Fix argojawn benchmark.

Turns out I hadn't actually enabled it properly. Ugh. Unfortunately
it looks like using Jawn's parser wasn't the magic bullet I thought.

At this point, Jawn is faster for maybe 3 reasons:

1. Deferring parseInt/parseDouble
2. More compact array/object representations
3. Possibly (?) parsing code

In any case, #1 is definitely the most significant. If argonaut used a similar
deferral strategy it could get a lot faster even without Jawn's parser. So,
sharing the parser may not really be as useful as I originally thought.