Solution 1:

rack-1.3.5 is the version number on the current copy, but rack (~> 1.1.0) is the version constraint that's been specified - they call this the "Pessimistic Version Constraint", and it's preventing anything newer than 1.1.x from fulfilling this dependency.

You'll need an older version of rack, or, maybe this "1.1 (Release: 1.3)" will work just fine - you could just try changing the version spec to >= instead of ~>, and see if it'll load.