Set up package-lock support for greenkeeper

.travis.yml

@@ -11,8 +11,15 @@ env:
 cache:
   directories:
   - "$HOME/.npm"
-install: npm ci
+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:
+- if [[ ${TRAVIS_BRANCH:0:11} = greenkeeper ]]; then npm install; else npm ci; fi
+before_script: greenkeeper-lockfile-update
 script: npm run $NPM_SCRIPT
+after_script: greenkeeper-lockfile-upload
 jobs:
     include:
     - env: NPM_SCRIPT=lint