use yarn in travis

.travis.yml

@@ -5,9 +5,9 @@ os:
   - linux
   - osx
 
-cache:
-  directories:
-    - $HOME/.npm
+# cache:
+#   directories:
+#     - $HOME/.npm
 
 notifications:
   email: false
@@ -33,20 +33,21 @@ before_install:
   - git submodule update --init --recursive
   - git clone --depth 1 https://github.com/creationix/nvm.git ./.nvm
   - source ./.nvm/nvm.sh
-  - nvm install 7.9.0
-  - nvm use 7.9.0
-  - npm config set python `which python`
+  - nvm install 8.9.1
+  - nvm use 8.9.1
+  - npm i -g yarn
+  # - npm config set python `which python`
   - if [ $TRAVIS_OS_NAME == "linux" ]; then
       export CXX="g++-4.9" CC="gcc-4.9" DISPLAY=:99.0;
       sh -e /etc/init.d/xvfb start;
       sleep 3;
     fi
   # Make npm logs less verbose
-  - npm config set depth 0
-  - npm config set loglevel warn
+  # - npm config set depth 0
+  # - npm config set loglevel warn
 
 install:
-  - ./scripts/npm.sh install
+  - yarn
 
 script:
   - node_modules/.bin/gulp hygiene