use before_last_save for rails >= 5.1

[kbrock]

The {attribute}_was has changed. there are now 2 different methods

changed the code according to whether it was called in the
after save (needed to use _before_last_save) OR
before save/destroy callback (still use _was)

This will remove rails 5.1 deprecation warnings

This is in response to #379 from @jjuliano
Thanks for the help @khustochka
Thanks for the PR @Bramjetten

if you all could give a quick peek, that would be appreciated

Fixes #372

[coveralls]

Coverage decreased (-0.2%) to 98.108% when pulling ef25574 on kbrock:51_warnings_pt2 into 9f226a4 on stefankroes:master.