Temporarily disable automatic doc deployment

to restore successful build status. travis DOCS job on master failed due to permisstion issue: https://travis-ci.org/baidu/Paddle/jobs/159851334  @reyoung, please resolve the permission issue.

paddle/scripts/travis/docs.sh

@@ -59,5 +59,8 @@ chmod 600 deploy_key
 eval `ssh-agent -s`
 ssh-add deploy_key
 
+# Temporarily disable automatic doc deployment due to permission issue.
+exit 0
+
 # Push
 git push $SSH_REPO $TARGET_BRANCH