[travis] Only build against first and last llvm supported version
[travis] Only build against first and last llvm supported version

alexander255/rust-bindgen,bsteinb/rust-bindgen,alexander255/rust-bindgen,bsteinb/rust-bindgen,Twinklebear/rust-bindgen,alexander255/rust-bindgen,bsteinb/rust-bindgen,Twinklebear/rust-bindgen,Twinklebear/rust-bindgen,Twinklebear/rust-bindgen