Readme fixes

Hi Tor team,

In preparation for use on Measurementlab.net, I made some changes to the installation instructions in the readme. I took out references to outdated code based on feedback from art@torproject.org, and made fixes and readability changes based on my own testing. Note that everything below "Using Ooniprobe" is mostly untouched.

I'll look out for any comments or corrections and happily incorporate them.  Thanks!
The first install option should be the recommend one, that works for debian testing and it should just be `apt-get install ooniprobe`.

The following instructions should be the more complicated ones and it should be noted that they are not officially supported distro versions.