Update bootstrap-typeahead.js

When add a typehead to a scrollable element the placement (height) of the typeahead dropdown is calculated wrong, See: http://bootply.com/66845 (remove the javascript to see the problem)
The scrollHeight (int or function) option can be used to correct this problem.

Example for typeahead input in the Modal use:

     scrollHeight: function() { return $('.modal-body').scrollTop(); }