Change #!/bin/bash line to the more universal #!/usr/bin/env bash
Add config.ssh.shell = "bash -c 'BASH_ENV=/etc/profile exec bash'" to Vagrantfile. This supresses the harmless but annoying 'stdin: is not a tty' error that is generated every time a shell script provisioner is run.

Thanks @bradfeehan for your comment on this page: https://github.com/mitchellh/vagrant/issues/1673