[MRG] DOC FIX foating point issue

I fix the following floating point issue.
I am on 32 bit linux machine.

```
======================================================================
FAIL: Doctest: feature_selection.rst
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/ajoly/opt/python/lib/python2.7/doctest.py", line 2201, in runTest
    raise self.failureException(self.format_failure(new.getvalue()))
AssertionError: Failed doctest test for feature_selection.rst
  File "/home/ajoly/git/scikit-learn/doc/modules/feature_selection.rst", line 0

----------------------------------------------------------------------
File "/home/ajoly/git/scikit-learn/doc/modules/feature_selection.rst", line 203, in feature_selection.rst
Failed example:
    clf.feature_importances_
Expected:
    array([ 0.12604616,  0.07234783,  0.38787583,  0.41373018])
Got:
    array([ 0.12526045,  0.07364783,  0.38842821,  0.41266352])

>>  raise self.failureException(self.format_failure(<StringIO.StringIO instance at 0x9d726ac>.getvalue()))
 ```