feat(collapse): Bootstrap3 compatibility/bugfixes

Same issues as #934:
- Make code more concise (I could use help with this, as I've said)
- Remove now-unused fixUpHeight routine

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Short of this, tests are passing, and from what I've seen, we're getting
good behaviour from all browsers. So this is good.

See #934 for other work on this same problem, I'd like to get this merged
in, for the benefit of my own apps and everyone elses :]

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[Passing on Travis-CI](https://travis-ci.org/angular-ui/bootstrap/builds/11308361)