Add six to requirements.txt

**Issue on a Debian wheezy box**

```
[intg] :jenkins: /opt/jenkins_job_builder$ git rev-list --max-count=1 HEAD
666e9537f79f9e9f8c03b7033ffd6c635ac4221c

[intg] :jenkins: /opt/jenkins_job_builder$ sudo python setup.py install
running install
Requirement already satisfied (use --upgrade to upgrade): argparse in /usr/lib/python2.7
Requirement already satisfied (use --upgrade to upgrade): PyYAML in /usr/lib/python2.7/dist-packages
Requirement already satisfied (use --upgrade to upgrade): python-jenkins in /usr/local/lib/python2.7/dist-packages
Requirement already satisfied (use --upgrade to upgrade): pbr>=0.8.2,<1.0 in /usr/local/lib/python2.7/dist-packages
Requirement already satisfied (use --upgrade to upgrade): pip in /usr/lib/python2.7/dist-packages (from pbr>=0.8.2,<1.0)
Cleaning up...
running build
running build_py
running egg_info
writing requirements to jenkins_job_builder.egg-info/requires.txt
writing jenkins_job_builder.egg-info/PKG-INFO
writing top-level names to jenkins_job_builder.egg-info/top_level.txt
writing dependency_links to jenkins_job_builder.egg-info/dependency_links.txt
writing entry points to jenkins_job_builder.egg-info/entry_points.txt
[pbr] Reusing existing SOURCES.txt
running install_lib
running install_egg_info
removing '/usr/local/lib/python2.7/dist-packages/jenkins_job_builder-0.8.1.52.g666e953.egg-info' (and everything under it)
Copying jenkins_job_builder.egg-info to /usr/local/lib/python2.7/dist-packages/jenkins_job_builder-0.8.1.52.g666e953.egg-info
running install_scripts
Installing jenkins-jobs script to /usr/local/bin

[intg] :jenkins: /opt/jenkins_job_builder$ jenkins-jobs 
Traceback (most recent call last):
  File "/usr/local/bin/jenkins-jobs", line 6, in <module>
    from jenkins_jobs.cmd import main
  File "/usr/local/lib/python2.7/dist-packages/jenkins_jobs/cmd.py", line 24, in <module>
    from jenkins_jobs.builder import Builder
  File "/usr/local/lib/python2.7/dist-packages/jenkins_jobs/builder.py", line 27, in <module>
    import jenkins
  File "/usr/local/lib/python2.7/dist-packages/jenkins/__init__.py", line 51, in <module>
    import six
ImportError: No module named six
[intg] :jenkins: /opt/jenkins_job_builder$ pip install six 
Downloading/unpacking six
  Running setup.py egg_info for package six
    
    no previously-included directories found matching 'documentation/_build'
Installing collected packages: six
  Running setup.py install for six
    
    no previously-included directories found matching 'documentation/_build'
Successfully installed six
Cleaning up...

[intg] :jenkins: /opt/jenkins_job_builder$ jenkins-jobs 
usage: jenkins-jobs [-h] [--conf CONF] [-l LOG_LEVEL] [--ignore-cache]
                    [--flush-cache]
                    {update,test,delete,delete-all} ...
jenkins-jobs: error: too few arguments
```