_.sortBy errors on symbols.

[jdalton]

_.sortBy([1, Symbol()]);
// errors

This affects _.max and _.min too.

[jgonggrijp]

_.sortBy is for when you can reduce your values to a single string or number so that you can meaningfully apply the < operator. If you need to do something more fancy, pass your own custom comparator function to Array.prototype.sort instead. Note that this function is also available in Underscore OOP notation and in chains as _(yourArray).sort.

Related to #2848.