Update python requirements in README
Update python requirements in README

We use Python features introduced in v2.7, so mention that as a
minimum in the README.

Fixes #314

Change-Id: I792f47879a3840bf7bdc776018a155e3776286e2

indiereign/shaka-player,TobbeEdgeware/shaka-player,vimond/shaka-player,russitto/shaka-player,TobbeEdgeware/shaka-player,brightcove/shaka-player,shaka-project/shaka-player,ustudio/shaka-player,russitto/shaka-player,tvoli/shaka-player,indiereign/shaka-player,Ross-cz/shaka-player,vimond/shaka-player,TheModMaker/shaka-player,ustudio/shaka-player,indiereign/shaka-player,vimond/shaka-player,Ross-cz/shaka-player,russitto/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,TheModMaker/shaka-player,TobbeEdgeware/shaka-player,brightcove/shaka-player,TheModMaker/shaka-player,Ross-cz/shaka-player,tvoli/shaka-player,TheModMaker/shaka-player,tvoli/shaka-player,baconz/shaka-player,shaka-project/shaka-player,baconz/shaka-player,ustudio/shaka-player,baconz/shaka-player,ustudio/shaka-player,brightcove/shaka-player