Fix broken Slack badge
Fix broken Slack badge

After DNS settings were changed a while back, the previous badge is no longer available at http://slack.dita-ot.org, which previously pointed to a self-hosted app on Heroku, but now redirects to the native Slack invite form at https://dita-ot.slack.com.

This replaces the old badge with an SVG variant from https://shields.io.

Signed-off-by: Roger Sheen <a61a1add71dadc74037ae5e44ee30713db9a7451@infotexture.net>

infotexture/dita-ot,dita-ot/dita-ot,drmacro/dita-ot,dita-ot/dita-ot,drmacro/dita-ot,infotexture/dita-ot,infotexture/dita-ot,dita-ot/dita-ot,infotexture/dita-ot,drmacro/dita-ot,infotexture/dita-ot,dita-ot/dita-ot,shaneataylor/dita-ot,drmacro/dita-ot,dita-ot/dita-ot,shaneataylor/dita-ot,drmacro/dita-ot,shaneataylor/dita-ot,shaneataylor/dita-ot,shaneataylor/dita-ot