README: Update preferred pip installation method

From [pip Documentation](https://media.readthedocs.org/pdf/pip/1.5.X/pip.pdf) (v1.5.1)
> Beginning with v1.5.1, pip does not require setuptools prior to running get-pip.py. Additionally, if setuptools (or distribute) is not already installed, get-pip.py will install setuptools for you.

The installation method seems to be a little outdated. Also, it might be annoying for some users to read and manually type from an image file. I found [this](http://stackoverflow.com/q/5585875) related discussion on SO and thought it would be appropriate to change the link to the [best answer](http://stackoverflow.com/a/17601159) in the discussion. I was going to link from the official pip website but unfortunately it didn't look clear enough.