Use gopkg.in/check.v1 instead of launchpad.net/gocheck

gocheck package moved to github and should be referenced as gopkg.in/check.v1
https://github.com/go-check/check
`cursor.One` should work without checking `IsNil` on a user side.

`cursor.One` is a higher level funcion to get a single element from the result
 set. It already returns `ErrEmptyResult`, so to be consistent with a
function responsibility it should also return that error even when user
didn't check `IsNil` before performing `One`.