Fix replaceFileIfMissing return value.
Bindings gen touches output Rust files to prevent it from running every time.
auto merge of #574 : metajack/servo/bindgen-deps, r=jdm

This touches the `.rs` files even if they don't change. This will cause libscript to rebuild sometimes even when it doesn't have to. This seems better than having bindgen run every time as the case where CodegenRust.py changes but doesn't change the output `.rs` seems like it should be rare.

r? @jdm