Floor the rating for fedmsg messages.

[ralphbean]

This, like in fedora-infra/fedmsg#201, is just to reduce erroneous fedmsg validation failures. Without this, the rating gets encoded on one machines as 0.9111111111112 on one machine and as 0.91111111111 on another. Those are not the same strings, and so the signature computation produces different signatures -- recipients think that the message is "invalid".

[pypingou]

:+1: :)

[ralphbean]

Thanks @pypingou!