build(config): update Travis configuration

.travis.yml

@@ -1,10 +1,13 @@
 after_script: greenkeeper-lockfile-upload
 before_install: npm install npm@latest --global
 before_script: greenkeeper-lockfile-update
+cache:
+  directories:
+    - $HOME/.npm
 env:
   global:
     - secure: fZE97oyux66HNZPAeqUA+CZN3HzsggrTMu/nU3yhmPcFHNkuE61IF562VwCkC7O4yIGQcWjI3HUUd/meOwOMAvLpV+wXtN9YNMWOOqsERM4tmD/xjQUNdWzlHfXlUcMp2wlv13uSxnoicI9zFKE63rO4JtOptKdVHCuEXMGdZ30cKIQ2hD3plFV42PzjGfKKS4l3Rj549bcqPY3v23koqgHGlhXeyBZTDMXmWtUo5w9DrrKfgwnTTukoeFzHvPTImDvh2s9H/mkXydzWWJk7KoVluYfYL+BE4Wmi3tKqo4sIakurHeeJ9a/6jt5gE4x4Yxx58n859Eg8LOBRLCFr40lImMnODo2ut1gz5M/iYniy6355vRQDvIVrZPQXxzWyac8fIeDBecHS0NoO8MUzaZa+dDzfldCFZ4zPqZC1ALAuYnMXx5+nt8s7RxGGLB/sZNBmWbqbhYvbakSd66Ly4IuBDre0fLeZ0VBttp4LvZiOVdmTbDD5qN8qIe368LEMNly+ZiOEOMgzOdU+W4shz+XlaBG+m17DkmjKnbatMSS3XTuRAQ2WC3ak1fR5mO7iTZuDVTBk40nbwmugD9MLH0APOQ3oCb6fntucyGAvtJCx4IvdcdLMLhI9/CfGTOk6py4GFq1HR1W8uN992xqPuj/fK7Y7JrOAESYVppvtzVk=
-install: npm ci || npm i
+install: npm ci || (rm -rf node_modules && npm install)
 language: node_js
 node_js: 6
 sudo: false