As you are using the PPA and not the repos (I can tell both by version numbers *and* your `apt-cache` output), I've marked this as "Invalid" against the Ubuntu repositories' nginx version, and repointed it to the appropriate project here on Launchpad.
As you are using the PPA and not the repos (I can tell both by version numbers *and* your `apt-cache` output), I've marked this as "Invalid" against the Ubuntu repositories' nginx version, and repointed it to the appropriate project here on Launchpad.