fix rails 4 Gemfile.lock under bundler 1.7

bundler 1.7 @sources no longer responds to include?. Conveniently,
the method we're overriding no longer calls include?

Change-Id: I5362b28a6819a4de9bc1e5de25b4753a8d77d9e1
Reviewed-on: https://gerrit.instructure.com/41510
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>