Remove strongly held target from kvo targets hash

This issue seems to also be resolved in this PR(https://github.com/rubymotion/BubbleWrap/pull/361), but has not been merged in yet. This PR resolves only the issue I ran into. I think keeping a strong reference in @targets is okay, as long as it is removed when no longer needed. The way it currently is, the target object will only be released when the observing object is also released(releasing @targets)