Add rage-sequel

[rsamoilov]

The PR adds the rage-sequel test, which is a variation of the existing rage test using the Sequel ORM. It follows the example of the existing sinatra and sinatra-sequel tests.

[rsamoilov]

cc @p8

[p8]

@rsamoilov Nice!
The non database json and plaintext calls are probably similar to the regular rage tests?
For sinatra we don't make those calls for sequel to reduce the total test time:
https://github.com/TechEmpower/FrameworkBenchmarks/blob/master/frameworks/Ruby/sinatra-sequel/benchmark_config.json

[rsamoilov]

Thanks, fixed!