models: Default Update.critpath to False

Turns out that a default is necessary if we want to run queries likes
this:

    db.query(Update).filter_by(critpath=False)

The above will not return the right updates, because they will default
to None rather than False.
alembic: Drop leftover comments

I forgot them in when alembic automatically generated the script.
alembic: Migrate the Update.critpath to its proper value
Merge pull request #19 from bochecha/critpath_model

Some more critpath stuff in the model