Resolve cyclic dependency with opencensus

[kernle32dll]

While looking at golang-migrate/migrate#202, I got stuck at google.golang.org/api.

While I suppose a cycling dependency is not a problem directly with go modules, it ultimately leads to including go.opencensus.io@v0.19.1, which then causes golang/lint#436 trough an old version of google.golang.org/genproto.

To be honest - I have no idea who should fix this, or how. Even if go.opencensus.io would update to google.golang.org/api@v0.3.0, that would still cycle back:

<start here> google.golang.org/api@v0.3.0
google.golang.org/api@v0.3.0 go.opencensus.io@v0.19.2
go.opencensus.io@v0.19.2 google.golang.org/api@v0.2.0
google.golang.org/api@v0.2.0 go.opencensus.io@v0.19.1
go.opencensus.io@v0.19.1 google.golang.org/genproto@v0.0.0-20181219182458-5a97ab628bfb

[jeanbza]

Thanks - we're actively working on this issue. Please continue to track in googleapis/google-cloud-go#1359.