Remove -Z install-upgrade for cargo-audit

This is now the default since rust-lang/cargo#7560, still need to use nightly for `-Z minimal-versions` though.

.travis.yml

@@ -16,7 +16,7 @@ matrix:
     cache: cargo
     rust: nightly
     install:
-    - cargo install -Z install-upgrade cargo-audit
+    - cargo install cargo-audit
     script:
     - cargo generate-lockfile -Z minimal-versions
     - cargo audit