Fix PySpark unit tests on Python 2.6

This commit allows PySpark's unit tests to run on Python 2.6.  A number of Linux distros (including CentOS 6.4, which powers the AMP Lab's Jenkins servers) still use Python 2.6 by default.

I also added logging to the `run-tests` script to redirect the test output into a file.

I updated the Jenkins pull request builder to run these tests, so you should see PySpark output at the end of the test log.