SPARK-1917: fix PySpark import of scipy.special functions

https://issues.apache.org/jira/browse/SPARK-1917

Author: Uri Laserson <laserson@cloudera.com>

Closes #866 from laserson/SPARK-1917 and squashes the following commits:

d947e8c [Uri Laserson] Added test for scipy.special importing
1798bbd [Uri Laserson] SPARK-1917: fix PySpark import of scipy.special

(cherry picked from commit 5e98967b612ccf026cb1cc5ff3ac8bf72d7e836e)
Signed-off-by: Matei Zaharia <matei@databricks.com>