Try to fix intermittent test failures

Travis is showing intermittent failures in
`test_cargo_compile_path_deps::path_dep_build_cmd` and
`test_cargo_compile_git_deps::git_dep_build_cmd` (added in #561 and #563).

I haven't managed to reproduce these failures locally, but I suspect the tests
are timing-sensitive.  This tries to guarantee that the timestamps during the
two operations can't be the same.