auto merge of #506 : alexcrichton/cargo/sooner-lockfile, r=wycats

It's quite annoying if you update a dependency, but it takes you awhile to get
the dependency building. Previously the dependency graph would have to be
updated each time because the lockfile was only written *after* a successful
build.

Other tools like `cargo generate-lockfile` will already generate a lockfile at
any time, so just make it easier by moving it up in the compilation process.