github-bot: add IPs of Jenkins workers

Providing `$JENKINS_WORKER_IPS` environment variable at startup will
activate the whitelist implemented in the bot, validating who's allowed
to pushed Jenkins job updates as inline PR statuses on github.com.

Refs nodejs/github-bot#142

Author: phillipj

setup/github-bot/resources/environment-file

@@ -9,3 +9,4 @@ LOGS_DIR=/home/{{server_user}}/logs
 JENKINS_API_CREDENTIALS={{envs.jenkins_api_credentials}}
 JENKINS_JOB_URL_CITGM={{envs.jenkins_job_url_citgm}}
 JENKINS_BUILD_TOKEN_CITGM={{envs.jenkins_build_token_citgm}}
+JENKINS_WORKER_IPS=147.75.69.113,147.75.73.189