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.

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.
Merge pull request #3290 from olivierlacan/patch-1

Add release dates to each version in CHANGELOG