Fix gson / opencensus bazel naming mixup

[duelinmarkers]

In 72b9ee2 (see also #3328, #3338), gson kept its old name while opencensus got what should have
become gson's new name instead of a fixed opencensus name.

I think this resolves that.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

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[duelinmarkers]

I signed it!

[googlebot]

CLAs look good, thanks!

[ejona86]

Tehehe. Oops.

[ejona86]

Okay to test

[duelinmarkers]

Could we have 5c902c9 and 7d0229a cherry-picked onto the v1.6.x branch so the first 1.6 release will be useable from bazel projects conveniently? I can submit a PR for this if it would be helpful. Thanks.

[ejona86]

@duelinmarkers, yeah, I think it makes sense to backport them. I've added the backport label for us to backport them before the release.

[pgr0ss]

Thanks @duelinmarkers for fixing my mistake. Looks I screwed up my scripting for bulk updating.