Build failure in osx 1.20

[fiveop]

The build problem occurs starting with build #1025:

dyld: Library not loaded: x86_64-apple-darwin/stage1/lib/rustlib/x86_64-apple-darwin/lib/librustc_driver-62abc69f.dylib

  Referenced from: /Users/travis/rust/bin/rustc

  Reason: image not found

I looked at the build logs. The only thing the logs reveal is a changed pip version (messages are slightly different), though that probably has nothing to do with this error.

[fiveop]

This stopped working between builds 1024 and 1025:

https://travis-ci.org/nix-rust/nix/jobs/164470714 (2016-10-02T18:43:19Z)
https://travis-ci.org/nix-rust/nix/jobs/165123419 (2016-10-05T02:22:39Z)

The logs differ in no significant way till the error occurs.

[fiveop]

#444 makes it unnecessary to look at this.