Breaking change in dash: Replace use of -max-by

Breaking changes in Dash 2.0:

- The `-min-by` and `-max-by` now take a comparator function to sort by.

- The `-min` and `-max` functions are no longer variadic, but take a list to be more in line with the other dash functions.

Please see https://github.com/magnars/dash.el/issues/40 for reasoning.

Unfortunately we don't really get to choose our version of a package in Emacs, so this change is forced on you. I'm sorry about that. Breaking changes really suck.