Update urllib to 1.8 (8a8c601bee)

I noticed this version of urllib3 was attempting to use RC4 as the cipher, and SSL connections on using pyOpenSSL would fail to servers that did not support RC4. Update to v1.8 version of urllib3 to get the new cipher settings pushed in https://github.com/shazow/urllib3/commit/2088570a293df42b1623dd74fcff0174d0565af5.