Wedged sccached timing out CI builds

Some builds have gotten stuck when compiling LLVM which I suspect is due to sccache getting wedged. Some example logs:

* https://travis-ci.org/rust-lang/rust/jobs/191132811
* https://travis-ci.org/rust-lang/rust/jobs/189053722
* https://travis-ci.org/rust-lang/rust/jobs/189889110

Unfortunately I don't have a lot more information other than that :(

cc @luser 