blog: Post for v0.11.9
events: fix TypeError in removeAllListeners

Check that `listeners` is actually an array before trying to manipulate it
because it won't be if no regular event listeners have been registered yet
but there are 'removeListener' event listeners.
doc: clarify child_process error behaviour

Clarify that an 'error' event may or may not be followed by an 'exit'
event and that it's not safe to make assumptions either way.
tls: add serialNumber to getPeerCertificate()

Add a 'serialNumber' property to the object that is returned by
tls.CryptoStream#getPeerCertificate().  Contains the certificate's
serial number encoded as a hex string.  The format is identical to
`openssl x509 -serial -in path/to/certificate`.

Fixes #6583.
blog: npm outage postmortem
Merge remote-tracking branch 'upstream/v0.10'