Merge pull request #1254 from swcarpentry/slack-update

Slack update

_config.yml

@@ -63,7 +63,7 @@ linkedin_url    : "https://www.linkedin.com/grp/home?gid=8279689"
 rss_url         : "http://software-carpentry.org/feed.xml"
 twitter_name    : "@swcarpentry"
 twitter_url     : "https://twitter.com/swcarpentry"
-slack_signup_url: "https://swc-slack-invite.herokuapp.com/"
+slack_signup_url: "http://slack-invite.carpentries.org/"
 msl_url         : "http://mozillascience.org"
 store_url       : "http://www.cafepress.com/swcarpentry"
 pad_url         : "http://pad.software-carpentry.org"

_posts/2015/09/2015-09-22-instructor-helper-retreat.html

@@ -64,7 +64,7 @@ <h2>Hosting a Site</h2>
 </p>
 <h2>Participating Remotely</h2>
 <p>
-If you're not able to attend one of the meetups we're arranging, we invite you to participate remotely via Google Hangouts on Air. All worldwide sessions will be broadcast live on YouTube, and remote participants are welcome to lead a session themselves. Send your session idea to the mentorship committee at <a href='mailto:[email protected]'>[email protected]</a> and we'll get you on the schedule! If you'd like to get some teaching practice in, contact us and we'll help connect you with the closest group, to whom you can teach over a remote connection. Also, we're encouraging everyone to <strong>join in the Slack chat room</strong> following <a href="https://swc-slack-invite.herokuapp.com/">these steps</a>.
+If you're not able to attend one of the meetups we're arranging, we invite you to participate remotely via Google Hangouts on Air. All worldwide sessions will be broadcast live on YouTube, and remote participants are welcome to lead a session themselves. Send your session idea to the mentorship committee at <a href='mailto:[email protected]'>[email protected]</a> and we'll get you on the schedule! If you'd like to get some teaching practice in, contact us and we'll help connect you with the closest group, to whom you can teach over a remote connection. Also, we're encouraging everyone to <strong>join in the Slack chat room</strong> following <a href="{{site.slack_signup_url}}">these steps</a>.
 </p>
 <h2>Come on out!</h2>
 <p>