Travis: switch to cache: npm and remove npm i. (#34)

This way Travis caches the proper dirs itself and also uses `npm ci` when possible.

Author: XhmikosR

.travis.yml

@@ -12,19 +12,14 @@ os:
   - linux
   - windows
 
-install:
-  - npm install
-
 script:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then npm run test:cov; fi
   - if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then npm run test; fi
 
 after_success:
   - if [[ "$TRAVIS_OS_NAME" == "linux" && "$TRAVIS_NODE_VERSION" = "10" ]]; then npm run coveralls; fi
 
-cache:
-  directories:
-    - node_modules
+cache: npm
 
 notifications:
   email: false