Update ShowExceptions to suit latest rack master

* Post 893a2c50 in rack/rack, the #pretty method used while generating
  the HTML error markup, implemented in Rack::ShowExceptions, returns a
  String instead of an Array. This change uses Array() to
  convert the exception string in both plain text and HTML modes
  to an array.
Merge pull request #907 from kgrz/fix-error-with-rack-master

Update ShowExceptions to suit latest rack master