Support go1.12 toolchain

[rickb777]

https://golang.org/doc/go1.12#compiler requires changes to the way the stack is analysed.

See

errors/stack.go

Line 136 in ba968bf

n := runtime.Callers(3, pcs[:])

in v0.8.1 for example.

[davecheney]

I believe this is no longer required, Keith rolled back the change to runtime.Callers

[davecheney]

See #200. I believe that Go 1.12 is supported by this package.