mlab volume factory occasional failure

This occurs occasionally and not all the time

Platform: OS X 10.11
VTK version: 7.0.0
Python: 3.4
Numpy: 1.11

```
$ python3 -m nose.core -v mayavi
...

======================================================================
FAIL: Test the mlab volume factory.
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/kit/ETS/mayavi/mayavi/tests/test_mlab_integration.py", line 324, in test_volume
vol2._ctf.get_color(0.25+range2*value))
File "/Users/kit/Envs/mayavi-py3/lib/python3.4/site-packages/numpy/testing/utils.py", line 912, in assert_array_almost_equal
precision=decimal)
File "/Users/kit/Envs/mayavi-py3/lib/python3.4/site-packages/numpy/testing/utils.py", line 733, in assert_array_compare
raise AssertionError(msg)
nose.proxy.AssertionError:
Arrays are not almost equal to 6 decimals

(mismatch 100.0%)
x: array([ 0.848521, 0.973162, 0. ])
y: array([ 0. , 1. , 0.186019])
-------------------- >> begin captured logging << --------------------
mayavi.core.registry: DEBUG: Engine [] named NullEngine2 registered
mayavi.core.registry: DEBUG: Engine named NullEngine2 unregistered
mayavi.core.registry: DEBUG: Engine named None unregistered
mayavi.core.registry: DEBUG: Engine named None unregistered
--------------------- >> end captured logging << ---------------------
```