Ensure script/validate-gofmt is executed with bash

While Travis does that, a user just running the script will get an
error.

Signed-off-by: Vincent Bernat <Vincent.Bernat@exoscale.ch>

script/validate-gofmt

@@ -1,3 +1,5 @@
+#!/bin/bash
+
 source "$(dirname "$BASH_SOURCE")/.validate"
 
 IFS=$'\n'