Fix weather segment display

Most likely a change on GitHub/Yahoo servers made it crash, when
non-robot ready result page started to appear inside YQL processor.
Using "raw.githubusercontent.com" URL helps and weather icon is again visible.

Closes: Lokaltog/powerline#949

Signed-off-by: Aleksandrs Ļedovskis <aleksandrs@ledovskis.lv>
Merge pull request #959 from aledovsky/bug/weather_segment_fix

Fix weather segment display