Fix bucket_size -> divide_by

[mholt]

Fixes #575. Not much to explain here, methinks. :)

[remi-stripe]

Thanks of making the change. This one is weird, I could have sworn it was called bucket_size instead of divide_by a week ago.

@brandur-stripe while it's a breaking change, it also did not work so we likely can release it as a minor version.

[brandur-stripe]

Thanks @mholt!

Remi — agree on not cutting a major for this change. I'll put a note about it in the changelog.

[brandur-stripe]

Released as 32.3.0.

[mholt]

Great! Thanks guys!