install always latest yarn version

.travis.yml

@@ -1,12 +1,17 @@
 language: node_js
+
 node_js:
-  - "10"
+  - 10
+
 cache: yarn
+
 before_install:
-  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.10.1
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s
-  - export PATH="$HOME/.yarn/bin:$PATH"
+  - export PATH=$HOME/.yarn/bin:$PATH
+
 install:
-  - yarn install --production=false
+  - yarn install
+  
 script:
   - yarn lint
   - yarn test