Add clippy to Travis

See @nvesly's original PR for merkle.rs at https://github.com/SpinResearch/merkle.rs/pull/33/

.travis.yml

@@ -21,6 +21,15 @@ matrix:
         - rustup component add rustfmt-preview
       script:
         - cargo fmt --all -- --write-mode=diff
+    - env:
+        - NAME='clippy'
+        # use env so updating versions causes cache invalidation
+        - CLIPPY_VERSION=0.0.200
+      rust: nightly-2018-05-14
+      before_script:
+        - cargo install clippy --version $CLIPPY_VERSION || echo "clippy already installed"
+      script:
+        - cargo clippy --all -- -D clippy
     - env: NAME='kcov'
       sudo: required # travis-ci/travis-ci#9061
       before_script:
@@ -30,7 +39,7 @@ matrix:
       script:
         - cargo kcov --print-install-kcov-sh | sh
         - cargo update # Creates `Cargo.lock` needed by next command
-        - cargo kcov --verbose --features dss --coveralls -- --verify --exclude-pattern=/.cargo,/usr/lib,src/proto
+        - cargo kcov --verbose --coveralls -- --verify --exclude-pattern=/.cargo,/usr/lib,src/proto
       addons:
         apt:
           packages: