Fix build on Go 1.9

Change-Id: Ie471f396b22cd88104b4f05dbcf3677c7ffd5f08

.travis.yml

@@ -2,6 +2,7 @@ language: go
 
 go:
     - 1.8.3
+    - 1.9rc1
 
 cache:
     directories:

gotestmain/gotestmain.go

@@ -156,6 +156,10 @@ func (matchString) WriteProfileTo(string, io.Writer, int) error {
     panic("shouldn't get here")
 }
 
+func (matchString) ImportPath() string {
+	return "{{.Package}}"
+}
+
 func main() {
 {{if .MainStartTakesInterface}}
 	m := testing.MainStart(matchString{}, t, nil, nil)