Urgent: Fix configuration parsing by replacing is-json with a try/catch

https://github.com/joaquimserafim/is-json/commit/d5c8a429cc953119fde1fbae636d499733392f9c broke configuration parsing. I caught this because my Travis builds were failing because superstatic was complaining. superstatic's own tests fail as well with a fresh install. I've replaced is-json with a more conventional try/catch.

@joaquimserafim, just a heads up about this.

No disaster here since the breaking change was easy to catch and it's not like it would have ever made it into divshot in production. I think it's important though to do a review of the dependencies and what semver operator is appropriate for each. It was also a mistake to replace a try/catch with a two month old, sparsely tested library, especially when try/catch would be in all ways a better solution for this case. 

Please don't think I'm pissed off — I'm thoroughly grateful for superstatic. Being able to replicate divshot's server locally with one line of code is incredible. It's a big reason why I'm moving all my applications over to Divshot.