Replace "launchpad.net/go-check" to "gopkg.in/check.v1"

Hi!

I tried `go test` in this project, but the error was caused because go-check is missing.
("launchpad.net/gocheck" has renamed to "gopkg.in/check.v1")

And please see this: [http://gocover.io/github.com/aibou/looper](http://gocover.io/github.com/aibou/looper)