Fix intermittent failures in path_dep_build_cmd and git_dep_build_cmd

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 fix the failure by sleeping for a second between the two operations.