Switch to yarn

.travis.yml

@@ -7,17 +7,18 @@ sudo: false
 dist: xenial
 
 cache:
-  npm: true
+  yarn: true
 
 before_install:
-  - npm config set spin false
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
+  - export PATH=$HOME/.yarn/bin:$PATH
 
 install:
-  - npm install
+  - yarn install --no-lockfile --non-interactive
 
 script:
-  - npm run lint:wrapper
+  - yarn lint:wrapper
-  - npm run lint:contracts
+  - yarn lint:contracts
 
 branches:
   only: