Pin old compiletest

[RalfJung]

See Manishearth/compiletest-rs#164

[RalfJung]

That's odd, what are these Windows failures?

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead? (see issue #27812)
 --> C:\Users\appveyor\.cargo\registry\src\github.com-1ecc6299db9ec823\rustc_term-0.0.1\win.rs:5:1
  |
5 | extern crate libc;
  | ^^^^^^^^^^^^^^^^^^
  |
  = help: add #![feature(rustc_private)] to the crate attributes to enable

Cc @alexcrichton any idea what might be happening here?

[oli-obk]

Clippy has the same problem: rust-lang/rust-clippy#3897 (comment)