Upgraded Devise dependency to be looser but with a minimum version of 3.2.4

Fixes #2473
Fixes #2594
Avoid using db:test:prepare to generate the dummy app.

Fixes #2629
Update the docs and add helpful notes.

Fixes #2577
Merge pull request #2635 from refinery/loose-devise-dependency

Upgraded Devise dependency to be looser but with a minimum version of 3.2.4