stupidgcm: skip tests on Go 1.4 and older

Quoting from the patch:

	We compare against Go's built-in GCM implementation. Since stupidgcm only
	supports 128-bit IVs and Go only supports that from 1.5 onward, we cannot
	run these tests on older Go versions.

internal/cryptocore/cryptocore_go1.5_test.go

@@ -7,11 +7,10 @@ import (
 	"testing"
 )
 
-// Native Go crypto with 128-bit IVs is only supported on Go 1.5 and up
 func TestCryptoCoreNewGo15(t *testing.T) {
 	key := make([]byte, 32)
 	c := New(key, false, true)
-	if c.IVLen != 12 {
+	if c.IVLen != 16 {
 		t.Fail()
 	}
 }

internal/stupidgcm/stupidgcm_test.go

@@ -1,3 +1,10 @@
+// +build go1.5
+// = go 1.5 or higher
+//
+// We compare against Go's built-in GCM implementation. Since stupidgcm only
+// supports 128-bit IVs and Go only supports that from 1.5 onward, we cannot
+// run these tests on older Go versions.
+
 package stupidgcm
 
 import (