Change Peek Threshold to 1/3rd of Viewport

Problem
-------

Currently the previous/next item is scrolled into view when peeking beyond 1/2 the height of the viewport. This is too much.

Solution
--------

Change this to threshold to 1/3rd of the viewport height.

Unit Tests
----------

Modified existing tests.

How To +10/QA
-------------

- Ensure Travis CI passes.
- `grunt qa`
- Open ScrollList demo
- Change to `peek` mode
- Drag the list so that you are peeking 1/3rd of the way into an adjacent item.
- On release, should see the adjacent item scroll into view.
- Drag so that you are not peeking more than 1/3rd of the way into an adjacent item.
- On release, should see the current item snap back to the default position in the viewport.

@lancefisher-wf @robbecker-wf @patkujawa-wf @georgelesica-wf @tomconnell-wf @todbachman-wf