travis: Only run cargo-audit on stable

… since there's no benefit in running it multiple times and cargo-audit
can't be built with older versions of rustc (at the time of writing,
cargo-audit has a minimum rustc version of 1.36.0)

.travis.yml

@@ -6,16 +6,22 @@ rust:
   - beta
 
 before_script:
-  - "rustup component add rustfmt"
-  - "rustup component add clippy"
-  - "cargo install --force cargo-audit"
-  - "cargo generate-lockfile"
+  - rustup component add rustfmt
+  - rustup component add clippy
+  - |
+      if [ "$TRAVIS_RUST_VERSION" == "stable" ]; then
+        cargo install --force cargo-audit
+      fi
+  - cargo generate-lockfile
 script:
-  - "cargo audit"
-  - "cargo fmt --all -- --check"
-  - "cargo clippy --all --all-targets --all-features -- -D warnings"
-  - "cargo build --all --verbose"
-  - "cargo test --all --verbose"
+  - |
+      if [ "$TRAVIS_RUST_VERSION" == "stable" ]; then
+        cargo audit
+      fi
+  - cargo fmt --all -- --check
+  - cargo clippy --all --all-targets --all-features -- -D warnings
+  - cargo build --all --verbose
+  - cargo test --all --verbose
 if: "type != push OR (tag IS blank AND branch = master)"
 notifications:
   email: false