Reduce rack version requirement

I would like to relax the rack requirement to allow for using this gem in Rails 2.3.X projects.

PR #110 pointed out that the gem only uses 2 methods from the Rack gem; `escape` and `parse_query`. A [search for rack](https://github.com/intridea/oauth2/search?q=rack) shows only calling the methods with a single parameter each. Both of these method signatures were introduced via commit https://github.com/rack/rack/commit/7ed819ad716661a67ada21d04e7815e493ccdb9d for version `0.1` of Rack.

This is a change of the gemspec's version for rack to be `'>= 0.1'`.