Updated json-schema version requirement to allow newer versions.

I'm not sure if there's any particular reason that this dependency was pinned @ 1.1.x, but I hit some conflicts when trying to write a composer plugin that itself depended on json-schema >1.3, as there have been some bug fixes to integer validation.

Composer tests seem good with json-schema 1.3.5, I'll see how they go under Travis CI.