Add hours per push to the 'try_usage' recipe

[ahal]

We'd like to get a sense of the average hours that get scheduled for each type of try selector. I tried implementing this in one single query once, but it ended up breaking AD if selecting too long a time period. Might need to gather hours data in a separate query.

Here is the recipe file: https://github.com/mozilla/active-data-recipes/blob/master/recipes/try_usage.py

Here is the query it runs: https://github.com/mozilla/active-data-recipes/blob/master/recipes/queries/try_commit_messages.query

[ahal]

Fixed by c9cd34b415af5f352b6c0931e789e7defcd5c6f0