Fix yarn installation

Author: pinkeen

.travis.yml

@@ -10,7 +10,7 @@ services:
 before_install:
   # Cloudfront edge lambdas support nodejs up to 10.x
   - docker pull mageops/aws-lambda-build:nodejs10.x
-  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version version-number
+  - curl -o- -L https://yarnpkg.com/install.sh | bash
   - export PATH="$HOME/.yarn/bin:$PATH"
 
 env: