Replace rcov with simplecov

rcov doesn't support Ruby 1.9+
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.
Upgrade to latest rack to make sure taps works

Fix taken from https://github.com/eric1234/taps/commit/fc2d4127d06c4d31015c435737e0e9c355a16c8d and applied as 4c84f78
Add sqlite3 as runtime dependency

Solves issue reported by many users on stackoverflow and
- https://github.com/ricardochimal/taps/issues/116
- https://github.com/ricardochimal/taps/issues/132
Update sinatra