Disable unit testing on rhel5 and older

Changes implemented in issue #72 and issue #83 break rpm builds on rhel5 or older. This pull request mitigates the issue of building on rhel5 by not running unittests during build. But we have not addressed if pynag unittests not functioning at all on python 2.4 is a good enough.

#### Running tests without unittest2 fails miserably

```
+ /usr/bin/python setup.py test
/usr/lib/python2.4/distutils/dist.py:236: UserWarning: Unknown distribution option: 'requires'
  warnings.warn(msg)
running test
Traceback (most recent call last):
  File "tests/build-test.py", line 24, in ?
    import test
  File "/root/pynag-0.6.1/tests/test.py", line 21, in ?
    import unittest2 as unittest
ImportError: No module named unittest2
```

#### pip installing unittest2 on rhel5 makes tests "a success" but none are run.. ;-)
```
# python setup.py test
/usr/lib/python2.4/distutils/dist.py:236: UserWarning: Unknown distribution option: 'requires'
  warnings.warn(msg)
running test

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK
```