Upgrade all deps to using build scripts
auto merge of #817 : alexcrichton/cargo/build-cmd, r=brson

This registers a new cargo snapshot as well as updates all dependencies to versions that are using build scripts. This means that overrides can now be used when looking for native libs for cargo!

This also modifies the behavior of `--enable-nightly` to add some extra configuration the buildbot needs to build cargo on CentOS.