(Not a real issue) not compatible with go 1.1.1

[Steven Agyekum]

Hey there I tried to use this middleware with go 1.1.1 but it is not working.

The Error: undefined sha256.Sum256 (line 10 and 11 in util.go)

So I updated my go to version 1.2.1

Now everything works fine.

As stated in the main readme of martini "go 1.1 and greater is required". So I was confused why it was not working 1.1.1