change hashbangs from /bin/bash to /bin/sh

Not all OSs have bash at /bin (FreeBSD has it in /usr/local/bin).

bin/amber

@@ -1,2 +1,2 @@
-#!/bin/bash
+#!/bin/sh
 node `dirname $0`/../cli/support/amber-cli.js $@

bin/amberc

@@ -1,2 +1,2 @@
-#!/bin/bash
+#!/bin/sh
 node `dirname $0`/../cli/support/amberc-cli.js $@