Move update from before to after_update

All tests pass, with one new test. This seemed a little too easy, so perhaps it should be tested by a few people. I will use it and comment back here on any problems. 

[Line 734 in `tests/unit/model_tests.rb`](https://github.com/straydogstudio/paper_trail/blob/master/test/unit/model_test.rb#L734) had to be changed because line 728 now creates a version within the expected range.