Remove support for Rails 3.2

The maintenance policy for rails is to support the last two release
series'. This would be 4.1.x and 4.0.x at this point.

Clearance 2.0 drops support for 3.2
Update views to use 1.9 hash syntax
Remove all previous deprecated code
Remove allow_sign_up configuration option

This options is removed in the interest of limiting the number of ways to
accomplish the same or similar tasks. If you wish to customize clearance
routes, run the `clearance:routes` generator to turn off the internal
routes and copy them to the host application for customization.