Use Yarn instead of npm

Fixes #25

Author: maoberlehner

.travis.yml

@@ -2,6 +2,6 @@ language: node_js
 node_js:
   - "node"
 script:
-  - npm run lint
-  - npm test -- --maxWorkers=4
-  - npm run coveralls
+  - yarn run lint
+  - yarn test -- --maxWorkers=4
+  - yarn run coveralls

CONTRIBUTING.md

@@ -22,7 +22,7 @@ Pull requests are the greatest contributions, so be sure they are focused in sco
    # Assign the original repo to a remote called "upstream"
    git remote add upstream https://github.com/maoberlehner/vuex-map-fields
    # Install the tools necessary for development
-   npm install
+   yarn install
    ```
 2. Create a branch for your feature or hotfix:
    ```bash