Fix broken build

[faust64]

Following on #159

@annieweng points out we now need Go >= 1.9 to build klar.

[NexoMichael]

LGTM

[faust64]

Lately, I've had further issues building Klar.
Using the Dockerfile previously proposed in this PR, it would fail with the following error:

+ go get -d github.com/optiopay/klar
+ go build ./src/github.com/optiopay/klar
# github.com/stretchr/testify/assert
src/github.com/stretchr/testify/assert/assertions.go:1703:5: undefined: errors.Is
src/github.com/stretchr/testify/assert/assertions.go:1726:6: undefined: errors.Is
src/github.com/stretchr/testify/assert/assertions.go:1749:5: undefined: errors.As
src/github.com/stretchr/testify/assert/assertions.go:1766:7: undefined: errors.Unwrap
src/github.com/stretchr/testify/assert/assertions.go:1770:7: undefined: errors.Unwrap

Not quite understanding what was going on, I just tried to re-upgrade Go.
Arbitrarily picked 1.13.
"works on my laptop".