Add bumping catalog dependency to process of creating a release

Eventually we want to be able to have Tasks declare what versions they
are compatible with and test against those
(tektoncd/catalog#373) but in the meantime, it
seems reasonable to bump the version used in catalog tests every time we
do a release (and even once we have tektoncd/catalog#373,
we would want to be running tests against new versions as well - and this
could have the benefit of giving us information about issues in a release
early, before users notice them!)

Question: how does the person making the release ensure that this bump
didn't make anything start failing? I think we should still make this
part of the process before we figure this out entirely, but we might
want to create a cron or something that runs all the tests nightly, and
maybe have the build cop trigger the release manually to ensure there
aren't any problems?

tekton/README.md

@@ -30,6 +30,9 @@ To make a new release:
    ([see one of way of doing that here](https://github.com/tektoncd/plumbing/tree/master/tekton/resources/release#create-draft-release)).
 1. Add an entry to [the README](../README.md) at `HEAD` for docs and examples for
    the new release ([README.md#read-the-docs](../README.md#read-the-docs)).
+1. Update [the catalog repo](https://github.com/tektoncd/catalog) test infrastructure to use the
+   new release by updating the `RELEASE_YAML` link in
+   [e2e-tests.sh](https://github.com/tektoncd/catalog/blob/v1beta1/test/e2e-tests.sh).
 
 ### Run the Pipeline