ENH: Fail gracefully for version not found.

Let's try this again.

```
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/buildslave/nd-bb-slave-sparc-wheezy/statsmodels-py2_x-wheezy-sparc/build/venv/local/lib/python2.7/site-packages/statsmodels-0.5.0-py2.7-linux-sparc64.egg/statsmodels/tools/print_version.py", line 120, in show_versions
    print("virtualenv: %s (%s)" % (virtualenv.__version__,
AttributeError: 'module' object has no attribute '__version__'
```
http://nipy.bic.berkeley.edu/builders/statsmodels-py2.x-wheezy-sparc/builds/6/steps/shell_5/logs/stdio