Work with newer rubygems versions, use ActiveSupport 3 (fixes #11)

[ches]

Hey there,

Why don't we deal with these issues of wrong ActiveSupport versions being loaded by just updating the app to use AS 3? :-) Also, keep gembox from asploding if a newer version of rubygems is installed.

There's a haml and sinatra version bump commit at the tip of this branch that's not entirely necessary, so feel free to cherry pick if you don't want that of course.

[ches]

Just perusing old open pull requests... bump :-)