latest rustc; remove duplicate import
latest rustc; remove duplicate import

fix https://github.com/gfx-rs/gfx-rs/issues/238

mitchmindtree/gfx-rs,gfx-rs/gfx-rs,red75prime/gfx,sectopod/gfx-rs,tanderegg/gfx,alexcrichton/gfx,JohnColanduoni/gfx,davll/gfx,ColinKinloch/gfx,ColinKinloch/gfx,tanderegg/gfx,bvssvni/gfx-rs,tempbottle/gfx,Bastacyclop/gfx,JohnColanduoni/gfx,aepsil0n/gfx-rs