Replace MultiJson with our own vendored OkJson implementation.

Raven doesn't need to be fast but it HAS to work. We need control on the JSON
encoding to avoid serialization issues.
Avoid most encoding issues in our OkJson fork
Handle circular references when encoding the JSON object.
Mark the json_adapter configuration as deprecated
Sanitize circular dependencies with a string representation.

Circular dependencies and JSON serialization don't play well together.
Depending on the library it might throw a SystemStackTooDeep or even segfault
the interpreter.

Here we replace the circular depency with a "[...]" or "{...}" string
representation that should make it clear later on what happened.
squashme: Fix for old rubies
Merge pull request #118 from zimbatm/okjson

Use our own JSON encoder