Add package-lock

.travis.yml

@@ -98,10 +98,16 @@ addons:
       - ubuntu-toolchain-r-test
     packages:
       - g++-4.8
+before_install:
+# package-lock.json was introduced in npm@5
+- '[[ $(node -v) =~ ^v9.*$ ]] || npm install -g npm@latest' # skipped when using node 9
+- npm install -g greenkeeper-lockfile
 install:
   - sudo -H pip install -r requirements.txt
   - npm --production=false install
   - npm --production=false update
+before_script: greenkeeper-lockfile-update
+after_script: greenkeeper-lockfile-upload
 jobs:
   include:
   - stage: test