Improve yarn install for travis

* yarn install instead
* disable postinstall script

.travis.yml

@@ -5,8 +5,7 @@ node_js:
   - "6"
 
 cache:
-  directories:
-    - node_modules
+  yarn: true
 
 branches:
   only:
@@ -14,8 +13,8 @@ branches:
   # - master 
   - develop
 
-# before_install:
-#   - curl -o- -L https://yarnpkg.com/install.sh | bash -s
-#   - export PATH="$HOME/.yarn/bin:$PATH"
+before_install:
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s
+  - export PATH="$HOME/.yarn/bin:$PATH"
 
-# install: yarn --frozen-lockfile
+install: yarn install --pure-lockfile --ignore-scripts