Jetpack: Add release dates to our changelog.

Merges in:
https://github.com/Automattic/jetpack/commit/ff56a996304e866f6d6b4b14e49be43e0f713c21
https://github.com/Automattic/jetpack/commit/d1bbbf86387b4976ca057aad2ca92f201a21ac7a



git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@986588 b8457f37-d9ea-0310-8a92-e5e31aec5664