2015-21.py: Fix bug in fight calculation function
2015-21.py: Fix bug in fight calculation function

Rounds were incorrectly truncated through integer division,
when they should have in fact be rounded up to the next integer.

For instance, given damage of 4 and hit points of 13, it will take
5 rounds to defeat the boss. But our previous method truncated 4.3
to 4, and so the result was incorrect.

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