Fix weather segment display

Most likely a change on GitHub/Yahoo servers made it crash, when
non-robot ready result page (was redirect there previously?) 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>