Fixed issue with isPast() returning true for now()

Resubmission of pull request #114 because it seems to have been lost.

@briannesbitt This pull request fixes the issue where isPast() would return true when the time you were testing was the same as Carbon::now(). This is because it was just using the negation of isFuture() which isn't necessarily correct. Hopefully this also fixes testing issues in the previous request.

Props to @tharumax for reporting this in #109