chore: install greenkeeper-lockfile

Author: janl

.travis.yml

@@ -1,6 +1,14 @@
 language: "node_js"
 node_js:
+  # keep latest first, so gk-lockfile can work its magic
+  # https://github.com/greenkeeperio/greenkeeper-lockfile#testing-multiple-node-versions
+  - 8.0
   - 4.0
   - 6.0
-  - 8.0
   - node
+before_install:
+# package-lock.json was introduced in npm@5
+- npm install -g npm@5
+- npm install -g greenkeeper-lockfile@1
+before_script: greenkeeper-lockfile-update
+after_script: greenkeeper-lockfile-upload