.travis.yml: Download rustc from rust-lang.org
.travis.yml: Download rustc from rust-lang.org

nicokoch/gl-rs,skligys/gl-rs,tomaka/gl-rs,jimarnold/gl-rs,jimarnold/gl-rs,bjz/gl-rs,bjz/gl-rs,neon64/gl-rs,nicokoch/gl-rs,neon64/gl-rs,skligys/gl-rs,tomaka/gl-rs