Version-up virtualenv to 1.11.6 

There's a bug in v1.10.1 of virtuanenv that impacts Windows and only 32-bit Python can work with 1.10.1, while for 64-bit Python virtualenv must be updated).. 
Steps to install Counterparty on 64-bit Python: https://wiki.counterparty.co/w/Counterparty_with_64-bit_Python_3.4#Install_Counterparty
If virtualenv 1.11.6 works fine on Ubuntu, can we move to that version?
Merge pull request #64 from rippler/patch-3

Version-up virtualenv to 1.11.6