Remove greenkeeper-lockfile

According to https://blog.greenkeeper.io/announcing-native-lockfile-support-85381a37a0d0 it’s unnecessary since Sept 2018?!

Also remove the update step, it hasn’t been necessary since we began pinning the scratch-* packages.

.travis.yml

@@ -15,17 +15,10 @@ env:
 cache:
   directories:
   - node_modules
-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:
 - npm --production=false install
-- npm --production=false update
-before_script: greenkeeper-lockfile-update
 script:
 - npm test
-after_script: greenkeeper-lockfile-upload
 before_deploy:
 - >
   if [ -z "$BEFORE_DEPLOY_RAN" ]; then