Run mutation testing in CI

.travis.yml

@@ -21,3 +21,11 @@ jobs:
         - rustup component add clippy
       script:
         - cargo clippy
+
+    - name: 'Rust: mutation testing'
+      rust: nightly
+      install:
+        - cargo install cargo-mutagen
+      script:
+        - cargo test    --package dns --features=dns/with_mutagen -- --quiet
+        - cargo mutagen --package dns --features=dns/with_mutagen