update rack to 1.6.0

There have been [a lot of changes to rack](https://github.com/rack/rack/compare/1.5.2...1.6.0) between 1.5.2 and 1.6.0 but all of the specs pass at least.  

The main fix I'm interested in is the XSS vulnerability in the default error app that uses Rack::ShowStatus.  The fixed issue is [here](https://github.com/rack/rack/pull/579)