Update JDK7 buildspec with update yarn gpg key command

Author: davidh44

release-resources/buildspecs/jdk7-build.yml

@@ -3,6 +3,7 @@ version: 0.2
 phases:
   install:
     commands:
+      - curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
       - apt-get update
       - pip install awscli==1.19.34 --upgrade --user
       - java -version