Interchange order of arguments in round method

[tweymuth]

This PR addresses issue #909.

[coveralls]

Coverage remained the same at 99.815% when pulling 9c51d5b on tweymuth:reorder-round-method-arguments into 048f70b on RubyMoney:v7_new.

[antstorm]

@tweymuth thank you for submitting the PR. What do you think would be the best way to safely roll this out?

[tweymuth]

Since you mentioned in issue #909 that this change should be included for the next major release, I was silently assuming that it would be OK to have such a breaking change. I thought that we would warn the user about the breaking changes in a post-install message or something like that. (That being said, I realize I forgot to update the CHANGELOG - I'll fix that in a moment.)

In addition, we could also include a deprecation notice in one of the next minor releases, stating that the round method will work differently in version 7. If you think this is a good option, I can take care of that and prepare another PR.