Hypothesis-jsonschema

- Performance improvement in calculating schema intersections

- Improved canonicalisation of `uniqueItems: false` case
- Improved canonicalisation of numeric schemas
- Reuse `jsonschema` validators during canonicalisation (performance improvement)

- Added a strategy for the `"color"` format
- Only apply string length filter when needed (small performance improvement)

- Fixed error in resolution of certain `$ref`\ s
- Improved canonicalisation of `anyOf` and `contains` keys

- Requires Hypothesis >= 5.3.0, for improved IP address strategies
- Better canoncialisation of array schemata

- Resolve local, non-recursive references via the `$ref` keyword.

This is the largest feature in a while, and for some schemata it is a breaking
change. It's also a fundamental part of the spec, so I'm OK with that!

Note that `hypothesis-jsonschema` will raise an explicit error rather than fetching
a remote resource via URI. If think your tests *really should* hit the network,
get in touch and we can discuss adding an off-by-default option for this.