Fixed a bug when stacktrace builds from an anonymous function

[marvell]

This code fixes a bug when a stacktrace builds from anonymous function. For example,

package main

func main() {
	func(){
		raven.NewStacktrace(0, 3, nil)
	}()
}

pack will be main.main and func will be func1, but should main and main.func1.

[marvell]

Guys, please, merge this MR.

[marvell]

@mitsuhiko please, take a look on it.

[mitsuhiko]

lgtk. Thanks!