added cargo-audit to travis

.travis.yml

@@ -3,15 +3,14 @@ language: rust
 cache: cargo
 
 before_cache: |
-    cargo install cargo-tarpaulin
-    cargo install cargo-update
-    cargo install-update --all
-
+  cargo install cargo-tarpaulin || echo "cargo-tarpaulin already installed"
+  cargo install cargo-update  || echo "cargo-update already installed"
+  cargo install cargo-audit  || echo "cargo-audit already installed"
+  cargo install-update --all
 
 # before_cache:
 #     - rm -rf /home/travis/.cargo/registry
 
-
 rust:
   - stable
   - beta
@@ -20,10 +19,15 @@ matrix:
   allow_failures:
     - rust: nightly
 
-script:
-- cargo clean
-- cargo build
-- cargo test
+script: |
+  cargo clean
+  cargo build
+  cargo test
+
+  # it's enough to run this once:
+  if [[ "$TRAVIS_RUST_VERSION" == stable ]]; then
+    cargo audit
+  fi
 
 after_success: |
   # this does require a -Z flag for Doctests, which is unstable!