Remove dependency on faraday_middleware/multi_json

Make streatcher work with latest faraday 0.9.0 and faraday_middleware 0.9.1, as multi_json has been removed from faraday_middleware.

Else you get this error

    gems/faraday-0.9.0/lib/faraday.rb:99:in `method_missing': undefined method `load_autoloaded_constants' for #<Faraday::Connection:0x000001010a4338> (NoMethodError)
    gems/stretcher-1.21.1/lib/stretcher.rb:8:in `<top (required)>'
    .....