Update to Rust master: Update process_builder to use the new Command builder API.

`Process::configure` is gone as of https://github.com/mozilla/rust/commit/046062d3bf0597fb2f40f7cacbbe4f438506247d

Update process_builder to use the new Command builder API.

All tests pass on Ubuntu 14.04 with the most recent Rust master
All tests except cargo_compile_with_invalid_code pass on Mac OS X 10.9 with the most recent Rust master. I believe the failure is unrelated to these changes. It is caused by the test creating the expected value from the directory from `os::tmpdir()` which is a directory in `/var/folders` and the spawned `cargo-compile` process having a CWD in `/private/var/folders`. On Mac OS X `/var` is symlinked to `/private/var`. Because of this the error output from `rustc` uses `/private/var/folders` and the expected output uses `/var/folders`. I wrote a minimal test case outside of cargo that demonstrates this behavior: https://gist.github.com/zvms/78c24f2eebf07ec2b51f. This is resolved by #15.