Update Gemfile source to silence Bundler warnings

Bundler wants use to use HTTPS.

Bundler considers it to be more secure.

Bundler must be appeased.

Change-Id: Ic21166760f84e91a61ec444106af5ca2a87926f2
Reviewed-on: https://gerrit.causes.com/29987
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>