Ensure Taps version of OkJson is used to properly encode data.

Taps uses a customized version of OkJson to exchange data between the client
and server. This customized version silently converts symbols to strings
as a convience (the original OkJson considers encoding symbols an error since
symbols are not a type supported by JSON).

Rack started using OkJson also to encode data but kept the original symantics.
Since the Taps::Server inherits from Sinatra::Base, and Sinatra::Base includes
Rack::Utils, and Rack::Utils is the namespace where Rack's version of OkJson
is installed this means the Rack version is now being used by taps instead of
the taps version. This means when symbols are encoded an error is generated.

To restore the original behavior this patch explicility references the taps
version of OkJson.

This fixes issue #128.