ci: use yarn 1.21.1

Author: nodkz

.travis.yml

@@ -10,7 +10,7 @@ notifications:
 node_js:
   - "10.19.0"
 before_install:
-  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.7.0
+  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.21.1
   - export PATH=$HOME/.yarn/bin:$PATH
 script:
   - yarn run test