chore: upgrade yarn

Author: boopathi

.travis.yml

@@ -6,12 +6,12 @@ language: node_js
 cache: yarn
 
 node_js:
-  - '10'
-  - '8'
-  - '6'
+  - "10"
+  - "8"
+  - "6"
 
 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 1.9.4
   - export PATH=$HOME/.yarn/bin:$PATH
 
 before_script:
@@ -34,10 +34,10 @@ stages:
 jobs:
   include:
     - stage: lint
-      node_js: '10'
+      node_js: "10"
       script: yarn lint
     - stage: publish
-      node_js: '10'
+      node_js: "10"
       script: 'echo "//registry.npmjs.org/:_authToken=${NPM_TOKEN}" > ~/.npmrc'
       deploy:
         provider: script