change rustc invocation to --no-trans

per mozilla/rust#3808

Currently `rustc --parse-only` only catches syntax errors. `--no-trans` ensures the code will compile. Unfortunately, this is slow. But, at the moment parse-only doesn't do much. Syntastic for vim is using --no-trans as of a39b397e6ae3b32d2a0497d0f4ddfec0479ad2bb, for example.