Weekly release schedule

Currently we release dask/dask and dask/distributed every two weeks on Friday by loose convention (we're happy skipping or adding a release based on need and availability).

I'd like for us to consider increasing this frequency to weekly, still on Fridays. I've chatted with @jrbourbeau (who seems to be doing most of the releasing these days) and he seems game.

I thought I'd list some concerns that might arise that we should be aware of:

1. This is an increased burden on maintainers
2. Changes will have less time to marinate in master in order to find issues with them

I'm personally ok with these. I thought I'd bring it up in case other folks had thoughts or concerns.

cc @TomAugspurger @jakirkham @quasiben