Upgrade backoff library

[sagikazarmark]

Unfortunately v2 of the used backoff library violates the implementation of Go Modules.

I noticed that during testing go1.13beta1 and GOPROXY:

curl https://proxy.golang.org/github.com/cenkalti/backoff/@v/v2.2.0+incompatible.info
not found: github.com/cenkalti/backoff@v2.2.0+incompatible: invalid version: +incompatible suffix not allowed: module contains a go.mod file, so semantic import versioning is required

Here is an issue discussing that: #76

So I upgraded the backoff library.

Unfortunately, anyone using a go proxy suffers from this with RC1:
https://circleci.com/gh/goph/logur/75