This causes packages and binaries built by Go 1.5 to look
out of date to Go 1.6 and vice versa, so that when you flip
between different Go versions but keep the same GOPATH,
the right rebuilding happens at each flip.

Go 1.4 binaries will also look out of date to Go 1.5,
but Go 1.5 binaries will not look out of date to Go 1.4
(since Go 1.4 doesn't have anything like this).
People flipping between Go 1.4 and Go 1.5 will still
need to use go install -a every time to flip to Go 1.4,
but not when they flip back to Go 1.5.

Fixes #6534.
Fixes #10702.

Change-Id: I0ae7f268f822d483059a938a4f22846ff9275b4c
Reviewed-on: https://go-review.googlesource.com/10760Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>