actionpack: Relax rack version limit

[jdbann]

Rack versions < 2.2.3 have security vulnerabilities and actionpack pins the rack version to ~> 1.6. Relaxing this should allow us to update in other places but closer testing in consuming apps will be necessary.

The changes in actionpack are from a combination of the following commits:

• 51211a9 - point at rack master
  • Rack constants were moved in v2
• 529136d - stop inheriting from Rack::Request
  • Rack request API changed in v2 so actionpack no longer inherits, including the helpers instead
  • Including earlier changes to use get_header instead of env seemed excessive, so the rename to fetch_header is not applied here