Remove cargo audit to speed up CI

.travis.yml

@@ -13,16 +13,8 @@ jobs:
 before_script:
   - rustup component add rustfmt
   - rustup component add clippy
-  - |
-      if [ "$TRAVIS_RUST_VERSION" == "stable" ]; then
-        cargo install --force cargo-audit
-      fi
   - cargo generate-lockfile
 script:
-  - |
-      if [ "$TRAVIS_RUST_VERSION" == "stable" ]; then
-        cargo audit
-      fi
   - cargo fmt -- --check
   - |
       if [ "$TRAVIS_RUST_VERSION" != "1.39.0" ]; then