use latest yarn

.travis.yml

@@ -17,7 +17,7 @@ env:
     - JOB=test
 
 before_install:
-  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 0.28.1
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
 
 script:
   # Babel doesn't build correctly with npm v2 (which comes bundled with Node v4)

circle.yml

@@ -10,8 +10,7 @@ machine:
 
 dependencies:
   pre:
-    # Version number can be removed once 0.28.x or higher is released as stable
-    - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 0.28.4
+    - curl -o- -L https://yarnpkg.com/install.sh | bash
   cache_directories:
     - ~/.yarn-cache
   override: