Pluck emails instead of find/collect

> Use pluck as a shortcut to select one or more attributes without
> loading a bunch of records just to grab the attributes you want.
>
>  – http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-pluck

Use `pluck` so that copy and pasted examples are faster.
Merge pull request #709 from garethrees/patch-1

Pluck emails instead of find/collect