Relaxed the 'json_pure' dependency to prevent Bundler conflicts in
user applications.

Older releases of the 'json_pure' gem should do fine for our purposes,
so there's no need to attempt to force 1.6.5+. SproutCore currently
has a 'json_pure (~> 1.4.6)' dependency, so I've made that the new
baseline to enable SPARQL use in Rails/SproutCore applications. This
could probably be further relaxed should that prove necessary for
other use cases.

See: https://github.com/flori/json/blob/master/CHANGES