Update multierror to latest as of now. Our version is very old and
dates back to Sep 2015[1]. Here, we aim to pick up a panic fix found in
n https://github.com/hashicorp/go-multierror/pull/11 (Dec 2016).
This is a purely hygiene maintenance change. I am unaware of any causes
of the panic in our current dependencies. Though, some private internal
libraries did rely on the "recent" behavior of go-multierror, and I
aimed to update here to ease our adoption of other libraries later.
[1] d30f09973e
