go/gcimporter15: update build tag for binary import (fix build for 1.6)

Also: renamed file to match 1.7 tag.

Fixes golang/go#14824.

Change-Id: Iea92292d93c1140e3396678bc37f50f0348e616a
Reviewed-on: https://go-review.googlesource.com/20694
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

go/gcimporter15/gcimporter17_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build go1.6
+// +build go1.7
 
 package gcimporter