Shim for _.uniq no longer needed

In more recent versions of Underscore (since Nov 4), the _.uniq function [contains essentially the exact same shim code](https://github.com/documentcloud/underscore/blob/master/underscore.js#L449-L453) you include in angular_underscore. This means that if you're running angular_underscore with, say, 1.4.4, you get this:

    _.uniq( [1, 1, 2 ])
    // RangeError: Maximum call stack size exceeded

So the shim is no longer needed here -- Underscore does it for you.