Auto merge of #1712 - alexcrichton:run-cargo-on-stable, r=brson

Auto merge of #1712 - alexcrichton:run-cargo-on-stable, r=brson

This commit moves Cargo onto 100% stable Rust now that the necessary fs features
have been stabilized. The tests were updated to understand when they're running
with a non-nightly compiler and disable all plugin-related tests. The only major
feature here is that the calls to `fs::set_file_times` were reimplemented
manually in the tests that Cargo has.

Cargo still requires a *nightly compiler* due to the stable features not having
made their way into the stable channel yet, but it will soon work on all of the
stable, beta, and nightly compilers once 1.1 is released!