Use dev version of Gravity temporarily

[natefoo]

Immediately after merging #13224, @dannon found a bug (galaxyproject/gravity#24) where Gravity threw an exception when a job_conf.xml exists but has no <handlers> section. I propose we run a dev version of Gravity temporarily until closer to the 22.01 release when we have worked out any other bugs that might appear (and maybe write some more tests) so that we don't have to release a bunch of buggy successive versions of Gravity during this period.

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