Add sign_releases call to circle.yml in stable branch (cherry-picked …

…from master)

circle.yml

@@ -50,6 +50,7 @@ deployment:
     owner: yarnpkg
     commands:
       - ~/ghr/ghr --username yarnpkg --repository yarn --token $KPM_CIRCLE_RELEASE_TOKEN v$(dist/bin/yarn --version) artifacts/
+      - curl https://nightly.yarnpkg.com/sign_releases?token=$SIGN_TOKEN
       - echo "//registry.npmjs.org/:_authToken=${NPM_TOKEN}" > ~/.npmrc
       - npm publish