fix Bundler patch in case gemfile is run multiple times

Change-Id: I544033693c8b9cdec16bf5ee31539c6f4904aafb
Reviewed-on: https://gerrit.instructure.com/30793
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>