Doesn't build on nightly after rustc_front refactor

[gyscos]

rust-lang/rust#32570

syntaxext_lint-0.1.4/src/lib.rs:8:1: 8:26 error: can't find crate for `rustc_front` [E0463]
syntaxext_lint-0.1.4/src/lib.rs:8 extern crate rustc_front;
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

[gyscos]

Fix is in #7

[mcoffin]

Fixed by 574b65a