twitter: use HTTPS for OAuth

fixes CNVS-10521

Twitter now requires SSL. See their announcement:
https://dev.twitter.com/discussions/24239

test plan:
  * create a test application at dev.twitter.com
  * enable the Twitter plugin with your test key
  * go to a user profile page and enable Twitter integration
  * verify that Twitter is added successfully

Change-Id: I918f1658d27af0c987b33d99320df7e72cbe01ba
Reviewed-on: https://gerrit.instructure.com/28884
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>