Make sure we don't miss tweets

[potash]

If something breaks in our app, or on Heroku's end, or on Twitter, and the bot doesn't get run successfully between 6 and 7pm local, as currently written we won't get a tweet that day.

One way to be robust to that would be to actually check our twitter history to see when the last tweet went up.

[potash]

This is super low priority

[beamalsky]

I'm setting up Sentry for error logging as part of #20, which will help with this.