Test failure when both wxpython and pyside are installed

This is what I see on Linux:

```
======================================================================
ERROR: Failure: ValueError (cannot set toolkit to wx because it has already been set to qt4)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/nose/loader.py", line 418, in loadTestsFromName
addr.filename, addr.module)
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/nose/importer.py", line 47, in importFromPath
return self.importFromDir(dir_path, fqname)
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/nose/importer.py", line 94, in importFromDir
mod = load_module(part_fqname, fh, filename, desc)
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/enable/savage/svg/tests/test_document.py", line 6, in
from enable.savage.svg.backends.kiva.renderer import Renderer as KivaRenderer
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/enable/savage/svg/backends/kiva/renderer.py", line 7, in
from enable.compiled_path import CompiledPath as KivaCompiledPath
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/enable/compiled_path.py", line 17, in
from toolkit import toolkit_object
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/enable/toolkit.py", line 47, in
_init_toolkit()
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/enable/toolkit.py", line 30, in _init_toolkit
traits_toolkit()
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/traitsui/toolkit.py", line 125, in toolkit
ETSConfig.toolkit = toolkit_name
File "/home/vagrant/.jaguar/envs/test-enable/lib/python2.7/site-packages/traits/etsconfig/etsconfig.py", line 213, in _set_toolkit
"already been set to %s" % (toolkit, self._toolkit)
ValueError: cannot set toolkit to wx because it has already been set to qt4
```