I ran
HUBOT_ZULIP_SITE=https://zulip.example.com HUBOT_ZULIP_BOT=hubot-bot@example.com HUBOT_ZULIP_API_KEY=your_key bin/hubot -a zulip
with values corrected for my local installation and got:
Zulip API Error: unable to verify the first certificate
[Fri Oct 16 2015 14:19:28 GMT+0200 (CEST)] ERROR Error: unable to verify the first certificate
at Error (native)
at TLSSocket. (_tls_wrap.js:929:36)
at TLSSocket.emit (events.js:104:17)
at TLSSocket._finishInit (_tls_wrap.js:460:8)