Remove yajl-Ruby support in favour of multi_json

[iainbeeston]

Right now json-schema supports three json backends, each with slightly different semantics:

1. The "json" gem
2. multi_json
3. yajl-ruby

However, yajl-ruby can be used via multi_json.

For the sake of simplicity, I'd suggest that we remove explicit yajl support in v3. Anyone who wants to use yajl can still use it via multi_json.

I may be missing some crucial bit of information here (about why json-schema supports both yajl and multi_json but if we're going to make some minor, backwards incompatible changes to clean up the code, I would suggest this one.

[iainbeeston]

This issue has been superseded by #195. Closing