Use sh instead of bash in .version.sh script

Fixes smallstep#1115

Author: maraino

.version.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 read -r firstline < .VERSION
 last_half="${firstline##*tag: }"
 if [[ ${last_half::1} == "v" ]]; then