Add release dates to each version in CHANGELOG

As I've described [here](http://keepachangelog.com), it's quite useful to associate release version numbers with dates to get situated in a change log. 

Hope you find this useful. By the way, Devise has been five years old for just a few days now!

![](http://media1.giphy.com/media/oLTrMZE0gCIgw/giphy.gif)

PS: While doing this I did notice that a release marked as yanked in the CHANGELOG was not showing up as yanked on RubyGems, hence #3289.