title: python2.7: [SSL: UNKNOWN_PROTOCOL] unknown protocol
body: <p>I'm trying to install ROS from source.<br></p>
  
  <p>When I execute the command of installation, I get such an error:<br></p>
  
  <pre><code>Traceback (most recent call last):
    File "/home/zyh/ros_catkin_ws/install_isolated/share/ros/core/rosbuild/bin/download_checkmd5.py", line 126, in &lt;module&gt;
      sys.exit(main())
    File "/home/zyh/ros_catkin_ws/install_isolated/share/ros/core/rosbuild/bin/download_checkmd5.py", line 73, in main
      urllib.urlretrieve('https://github.com/assimp/assimp/archive/v3.1.1.zip', dest)
    File "/usr/lib/python2.7/urllib.py", line 98, in urlretrieve
      return opener.retrieve(url, filename, reporthook, data)
    File "/usr/lib/python2.7/urllib.py", line 245, in retrieve
      fp = self.open(url, data)
    File "/usr/lib/python2.7/urllib.py", line 213, in open
      return getattr(self, name)(url)
    File "/usr/lib/python2.7/urllib.py", line 443, in open_https
      h.endheaders(data)
    File "/usr/lib/python2.7/httplib.py", line 1038, in endheaders
      self._send_output(message_body)
    File "/usr/lib/python2.7/httplib.py", line 882, in _send_output
      self.send(msg)
    File "/usr/lib/python2.7/httplib.py", line 844, in send
      self.connect()
    File "/usr/lib/python2.7/httplib.py", line 1263, in connect
      server_hostname=server_hostname)
    File "/usr/lib/python2.7/ssl.py", line 363, in wrap_socket
      _context=self)
    File "/usr/lib/python2.7/ssl.py", line 611, in __init__
      self.do_handshake()
    File "/usr/lib/python2.7/ssl.py", line 840, in do_handshake
      self._sslobj.do_handshake()
  IOError: [Errno socket error] [SSL: UNKNOWN_PROTOCOL] unknown protocol (_ssl.c:661)
  /home/zyh/ros_catkin_ws/install_isolated/share/mk/download_unpack_build.mk:37: recipe for target 'build/assimp-3.1.1/unpacked' failed
  make[3]: *** [build/assimp-3.1.1/unpacked] Error 1
  </code></pre>
  
  <p>I don't know how to solve this issue. Maybe it's because I worked behind a proxy? If so, how to make <code>urllib.urlretrieve</code> work behind the proxy?</p>