Travis permissions error: failed to write Cargo.lock

Let's monitor this error, and see if it happens again.

From https://trac.torproject.org/projects/tor/ticket/23576?replyto=24#comment:24

{{{ error: failed to write /home/travis/build/torproject/tor/tor-0.4.0.1-alpha-dev/src/rust/Cargo.lock

Caused by: failed to open: /home/travis/build/torproject/tor/tor-0.4.0.1-alpha-dev/src/rust/Cargo.lock

Caused by: Permission denied (os error 13) }}} https://travis-ci.org/torproject/tor/jobs/486721901#L3634