Installing ungit on Debian Wheezy

Added installation guide for ungit on Debian Wheezy under "Known Issues", 
because Debian's supported packages through their packagemanager are too old.
It works fine for me on Debian Wheezy and at [#406](https://github.com/FredrikNoren/ungit/issues/406) FredrikNoren says:
>Only reason that version check thing is there is because I'm not actively testing it on < 1.8. I think we could change it so that when it's < 1.8 it still starts but shows a warning in the UI