Use `sort_by` instead of `sort_by!`

`sort_by!` and other mutator methods are blacklisted as of Ruby on Rails
4.1.0.rc2 and now yield `NoMethodError`.

References:
* https://github.com/rails/rails/commit/d4ee09c
Merge pull request #3064 from jgorset/fix/undefined-method-sort-by!

Use `sort_by` instead of `sort_by!`