Add pagination to /api/v1/activity/just_updated

Hi,

I've added pagination, using will_paginate, to the just_updated endpoint.
I kept the default page size (50), and simply added support for a page parameter.

Since the limit parameter for the Version.just_updated method is now unused, I've added a second commit to remove it.
Feel free to ignore it if the parameter is useful for something I missed.

Thanks a lot!

PS: This change renders the documentation incomplete (see http://guides.rubygems.org/rubygems-org-api/#activity). I couldn't find out which steps to take to provide an update to the documentation along with the code.