Fix TestInventory class by explicitly using /bin/bash

Here is the problem:

```bash
$ make tests
PYTHONPATH=./lib nosetests -d -v
test_callback (TestCallback.TestInventory) ... /bin/sh: 1: source: not found
Traceback (most recent call last):
  File "/usr/bin/ansible-playbook", line 24, in <module>
    import ansible.playbook
ImportError: No module named ansible.playbook
FAIL
```
Note the `source: not found`

TestCallback.py:
```python
subprocess.call('source ../../hacking/env-setup 2>&1 >/dev/null; ansible-playbook -i "127.0.0.1," test_playbook.yml 2>&1 >/dev/null', shell=True)
```

`source` is a builtin function of bash and not available in e.g. `/bin/sh`. For whatever reason my system is using /bin/sh instead of /bin/bash. I think if the subprocess call depends on bash functionality, it should force the usage with `executable='/bin/bash'`

FYI: I'm using a Debian sid with rxvt-unicode, /bin/bash for the login shell and virtualenv for the ansible project checkout.