Python 2.6 is required for gyp

Document the problem (http://stackoverflow.com/questions/21155922/error-installing-node-gyp-on-ubuntu) and suggest a solution (use a Python version manager eg pyenv). This is especially helpful for Ubuntu 14.04 which comes with Python 2.7.
Merge pull request #597 from ethers/master

Python 2.6 is required for gyp