[fix] broken Go 1.9 compatibility (prebid#910)

This CL addresses Go 1.9 compatibility issue along with adding back Go
1.9 to travis testing setup to prevent such bugs.

Issue: prebid#895

.travis.yml

@@ -1,6 +1,7 @@
 language: go
 
 go:
+  - '1.9'
   - '1.10'
   - '1.11.1'
 

macros/macros.go

@@ -1,7 +1,7 @@
 package macros
 
 import (
-	"strings"
+	"bytes"
 	"text/template"
 )
 
@@ -19,11 +19,12 @@ type UserSyncTemplateParams struct {
 
 // ResolveMacros resolves macros in the given template with the provided params
 func ResolveMacros(aTemplate template.Template, params interface{}) (string, error) {
-	strBuilder := strings.Builder{}
-	err := aTemplate.Execute(&strBuilder, params)
+	strBuf := bytes.Buffer{}
+
+	err := aTemplate.Execute(&strBuf, params)
 	if err != nil {
 		return "", err
 	}
-	res := strBuilder.String()
+	res := strBuf.String()
 	return res, nil
 }