RecordArrays no longer tracking `deletedItems`

[conrad-vanl]

Noticed today that after Merge of #21 deletedItems in a hasMany recordArray is no longer being tracked, which breaks the patch style array/relation updates here and means you can't remove an item from a Parse Array (such as a hasMany array) and have that change be picked up by Parse.

https://github.com/clintjhill/ember-parse-adapter/blob/master/lib/ember-parse-adapter/serializer.js#L189

I was working on a few different ways to add this back, but couldn't determine a reliable method. It would be nice to not have to rely on tracking "deletedItems" in the record array, and instead do like a diff of the changedAttributes - but it looks like the changedAttributes method does not work properly for record array type attributes.

[mixonic]

Closing this in favor of #49, hunting for the same thing.