install: do require bash, for simplicity

Supporting generic sh would mean testing with at least bash and dash.
/bin/bash will pretty much be always available ?

[ci skip]

hledger-install/hledger-install.sh

@@ -1,4 +1,4 @@
-#!/bin/sh -e
+#!/bin/bash -e
 
 usage() {
   cat <<'HERE'