ci(npm): cache node modules in each CI config

.travis.yml

@@ -3,6 +3,10 @@ language: node_js
 node_js:
   - 8
   - 9
+# keep the npm cache to speed up installs
+cache:
+  directories:
+  - "$HOME/.npm"
 after_success:
   - npx nyc report --reporter=lcov | npx codecov
   - npm run cov:check