Drop support for IE < 11

Tie-in issue to Backbone issue [#3828](https://github.com/jashkenas/backbone/issues/3828).

On Jan 12, 2016 MS is [essentially dropping support for IE8](https://support.microsoft.com/en-us/gp/microsoft-internet-explorer). With that in mind jQuery and Lodash are dropping IE8 support in their 3.0 and 4.0 releases. So let's discuss dropping it in the next major bump of Underscore.

Start by reviewing the linked Backbone issue and the support rationale in this discussion, then identify Underscore's current browser-support policy and any compatibility-related configuration or tests. Done means agreeing on the next-major-version scope and removing IE versions below 11 from the supported browser range.

Written by the indexing model from the issue text.