Use "uri-reference", not "url" or "uriref"

There is no way to tell a validator that a schema is
using draft-wright-json-schema-validation-00, which is the
only draft with a "uriref" format.  Validators either
interpret schemas as the previous draft (fge-04), which does
not have this concept at all, or the following draft (wright-01)
which has "uri-reference".

Since there are no options that will automatically work correctly,
using the later form seems to be the best.  Many validators allow
registering extensions, and we can just document that we are
using the wright-01+ syntax.  You often need to register format
handlers to get *any* format validation with many validators
anyway, so this does not seem overly burdensome.