Just use JSON

Instead of using MultiJson, can we just use JSON?

## Philosophy
We aren't terribly concerned about speed. The amount of JSON Ridley/Berkshelf are processing is nominal. And we also aren't that concerned about memory leaks and stuff because most tasks complete in less than 1 minute. So I don't really think the holy war on JSON implementations actually applies here.

## Rationale
The "lowest" Ruby supported is 1.9.2. Since 1.9.1, Ruby has come packaged with its own JSON implementation... JSON. It doesn't require an external gem or dependency - it just works. 

Instead of worrying about all these different JSON implementations on various platforms, etc, let's just use the built-in Ruby one.

## Fixes
1. reset/chozo#23
2. Human-readable lockfile format
3. Human-readable JSON formatter
4. Removes multiple dependencies from the gemspec