Automatic merge from submit-queue

Up to golang 1.6

A second attempt to upgrade go version above `go1.4`
Merge ASAP after you've cut the `release-1.2` branch and feel ready.
`go1.6` should perform slightly better than `go1.5`, so this time it might work
@gmarek @wojtek-t @zmerlynn @mikedanese @brendandburns @ixdy @thockin