Add go.mod support?

[andresvia]

Today when backoff is imported in a go.mod enabled module this happens to mymod go.mod (note the +incompatible).

Add a go.mod that makes backoff compatible with the GO module specification.

Thanks.

module mymod

go 1.12

require (
	github.com/cenkalti/backoff v2.1.1+incompatible
)