change organizers page to use https for twitter and github

[bridgetkromhout]

Expected behavior

Use https for twitter and github to prevent unneeded redirects.

Actual behavior

Currently using http on the organizer page. As @adrianmoisey put it in the PR, "both GitHub and Twitter are served on https. Linking to the http version causes a redirect."

Reproduction Steps

See the http links on https://www.devopsdays.org/events/2017-cape-town/contact/.

I checked and we're correctly using https on the CTA, footer, shortcodes, speaker page, talk page. This seems to be the only place twitter was http.

[mattstratton]

This was fixed in #594