golang-github-slack-go-slack release 0.11.3-3 for unstable

[dgit please-upload source=golang-github-slack-go-slack version=0.11.3-3]
[dgit distro=debian split --quilt=unapplied]
[dgit upstream-tag=v0.11.3 upstream=6f5eda2a6c1e50c8430bb07657bbe8be0def2b37]