Fix pgadmin bash (#227)

When trying to run this file, we'd get the error 

`./build-pgdmin: bad interpreter: /usr/bin/bash: no such file or
directory`

cloud/aws/pgadmin/build-pgdmin

@@ -1,4 +1,4 @@
-#!/usr/bin/bash
+#! /usr/bin/env bash
 
 # Builds the customized pgadmin Docker image.