Downgrade rack dependency to 1.1

I couldn't find any reason for limiting rack to 1.2+ (I didn't search very long for one either, truth be told).

This should allow support for Rails 2.3 apps (2.3.18 requires `rack ~> 1.1.0`).

Instructions :

* Manually `require 'mobvious'` in `config/environment.rb`
* In the same file, insert `config.middleware.use Mobvious::Manager`

:warning: I only tested this in one app, running rails 2.3.18 & rack 1.1.6.