Fix TravisCI config

.travis.yml

@@ -50,17 +50,15 @@ cache:
   directories:
     - node_modules
     - "$HOME/.nvm"
-    - "$HOME/.yarn"
     - "$HOME/.yarn-cache"
 
 # Use an updated Node runtime
 # See: http://entulho.fiatjaf.alhur.es/guias/how-to-use-node-along-with-other-language-on-travis-ci/
 install:
     - . $HOME/.nvm/nvm.sh
-    - nvm install 6.9.2
-    - nvm use 6.9.2
-    - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.8.1
-    - yarn install --no-progress
+    - nvm install 6.9.1
+    - nvm use 6.9.1
+    - version=0.18.1 && wget https://yarnpkg.com/downloads/$version/yarn-v$version.tar.gz && mkdir yarn && tar zxf yarn-v$version.tar.gz -C yarn && yarn/dist/bin/yarn install --no-progress
 
 # Clones WordPress and configures our testing environment.
 before_script: