internal: bump opencensus to non-existent future version

This CL is one in a line of CLs to remove github.com/golang/lint from
transitive deps. It is being done in live coordination with opencensus.

This commit will be tagged v0.3.1+alpha.

Change-Id: I36da0d5de3e0a75974dccda0550d8db131fdbd62
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/39671
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>
diff --git a/go.mod b/go.mod
index 4c5eec4..d96e650 100644
--- a/go.mod
+++ b/go.mod
@@ -2,7 +2,7 @@
 
 require (
 	github.com/google/go-cmp v0.2.0
-	go.opencensus.io v0.19.2
+	go.opencensus.io v0.20.1+alpha
 	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