Update Rack to 1.6.1

This fixes a bug where all form fields in a request are counted against the multipart file limit:
rack/rack#814

This is an issue in production - some publications and organisations have a large number of dynamically added nested forms. When the form is submitted, a 500 is returned because the number of fields exceeds the limit. It only affects forms posted with multipart, e.g. ones that contain file fields.

Zendesk: https://govuk.zendesk.com/agent/tickets/1037587
Story: https://www.pivotaltracker.com/story/show/94668330

Gemfile.lock

@@ -267,7 +267,7 @@ GEM
       pry (~> 0.10)
     quiet_assets (1.1.0)
       railties (>= 3.1, < 5.0)
-    rack (1.6.0)
+    rack (1.6.1)
     rack-accept (0.4.5)
       rack (>= 0.4)
     rack-cache (1.2)