set specific yarn version for travis CI (#340)

Author: jenweber

.travis.yml

@@ -12,7 +12,7 @@ cache:
   yarn: true
 
 before_install:
-  - curl -o- -L https://yarnpkg.com/install.sh | bash
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.0.1
   - export PATH=$HOME/.yarn/bin:$PATH
   - yarn global add bower