doc: sync CHANGELOG.md from master

Removes the known issue about known memory leaks. This was originally
changed in the commit that synced the changelog to master:

https://github.com/nodejs/node/commit/cded6e7993284c734d2ae9244dfc1e5c4cd4d976

PR-URL: https://github.com/nodejs/node/pull/2524


Reviewed-By: Jeremiah Senkpiel <fishrock123@rockemail.com>