go/gcimporter15: don't run 1.7-specific test under 1.8 (fix build)

TBR=adonovan

Change-Id: I310ff6e51d681b2e5c8288e73870ce7d68e45518
Reviewed-on: https://go-review.googlesource.com/27208
Reviewed-by: Robert Griesemer <gri@golang.org>
1 file changed