Merge pull request #36 from ruby-jp/update-invitation-link

Update slack invitation link

Author: hsbt

_config.yml

@@ -20,4 +20,4 @@ twitter:
   card: summary_large_image
   username: ruby-jp
 
-slack_invite_link: https://join.slack.com/t/ruby-jp/shared_invite/zt-1h1wub6jl-s37DRnh3lPDeyCt5REr3Yw
+slack_invite_link: https://join.slack.com/t/ruby-jp/shared_invite/zt-1x6ih6re9-rb_RdylYdnqrVplw278n4Q