The Go 1.6 release notes say that Go 1.7 will remove support
for the GO15VENDOREXPERIMENT environment variable,
making vendoring always on. Do that.

Change-Id: Iba8b79532455828869c1a8076a82edce84259468
Reviewed-on: https://go-review.googlesource.com/19615Reviewed-by: Ian Lance Taylor <iant@golang.org>