mod: switch to non-alpha future versions of opencensus

The experiment proved mostly successful: we are able to remove
github.com/golang/lint from our mod graph, but we not are able to
GO111MODULE=on go get -u ./... on this repo. Filing an issue in golang/go
for the remaining weirdness, for but now this is still forward progress.

This will be tagged 0.3.1.

Change-Id: I78a890a16175e544bc404d403932b452dbb27646
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/39711
Reviewed-by: Tyler Bui-Palsulich <[email protected]>

go.mod

@@ -2,7 +2,7 @@ module google.golang.org/api
 
 require (
 	github.com/google/go-cmp v0.2.0
-	go.opencensus.io v0.20.1-alpha
+	go.opencensus.io v0.20.1
 	golang.org/x/lint v0.0.0-20190301231843-5614ed5bae6f
 	golang.org/x/oauth2 v0.0.0-20190226205417-e64efc72b421
 	golang.org/x/sync v0.0.0-20190227155943-e225da77a7e6