Allow to upload a version with the same number as rejected versions (bug 750909)

Fixes [bug 750909](https://bugzilla.mozilla.org/show_bug.cgi?id=750909)
Regarding the values_list, I'd rather not: the `translations_for_field` needs a field, not just a value. I could modify it to only need an id, but then the `if field is None` will need to be done here instead of in `translations_for_field`.

What do you think?