Address discrepancy between minimum billable expectations vis a vis the handbook

[cantsin]

Reported via #tock.

I noticed that the 18F Handbook and Tock have two different minimum billable expectations for a 4-day work week. The 18F Handbook says bill at minimum 25.5 hours, but Tock says 26 hours. It’s a small difference, but wanted to mention it! I’m not sure which one is right either.

Tock rounds to the nearest whole hour (80% of 32 hours is 25.6 hours) and the handbook rounds to the nearest 0.5 hour. We should clarify this confusion, either by fixing Tock to permit approximate hours or updating the handbook.