use rails4 find_by method when available

In rails4, it looks like tanker will use `find_all_by_id` if `activerecord-deprecated_finders `is in your Gemfile but otherwise it will fall back to `find`.  `find` behaves differently than `find_all_by_id` because it will raise an exception unless all models are present.  To allow the current behavior to continue in a rails4, without requiring `activerecord-deprecated_finders` as a dependency, we can use the `where` method if it is available. I'm not actually sure why we wouldn't use `where` all the time.  The use of a dynamic finder was added with commit 02b623a2 and I couldn't tell from the commit message why this was done (maybe it was just done so it could be stubbed for testing).  I would argue that falling back to `find` prior to rails3 (or for some other ORM) isn't the right behavior either, but I'll leave that for others to deal with.  I have added a test that describes the behavior when none of the dynamic finder methods are available.

Thanks for your consideration,

Andrew