fix: travis-ci should use npm install instead of npm ci

.travis.yml

@@ -8,6 +8,7 @@ node_js:
 before_install:
   - npm install -g npm@5
   - npm install -g greenkeeper-lockfile@1
+install: npm install
 before_script:
   - greenkeeper-lockfile-update
   - sudo mount --make-shared /