Update installation-linux.rst

1. As noticed by the daily builds, we need pkg-config now:

https://code.launchpad.net/~thopiekar/+archive/kivy/+build/5469211/+files/buildlog_ubuntu-saucy-amd64.kivy_1.8.0-testing0%2B201401161533-1696-testing22~ubuntu13.10.1_FAILEDTOBUILD.txt.gz 

2. Replaced the usage of our PPA to upgrade Cython to Cython's own PPA