Use `make cargo-test` on Travis
Use `make cargo-test` on Travis

Forgot to update .travis.yml in my last commit, and now Travis jobs fail
because `cargo test` in root of the repository can't find Cargo.toml.

der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat