Remove clone method

If I recall correctly, before Rails 3, only `clone` was implemented in
active record. From Rails 3.1, `clone` and `dup` are both used, with
semantics that follows Ruby:

* clone only does a shallow copy, meaning that attributes are not
  copied, and if you change an attribute on one object, it also changes
  on its clone
* dup returns a new object with no id, but with a copy of the attributes
  (except created/modified at).

Globalize's `clone` seems to be based off the Rails 2 semantics. In
001548d I implemented the dup method, and I think globalize should no
longer implement clone.