fix path to RAILS_LTS file in Gemfile

fixes CNVS-12130

test plan:
- have a RAILS_LTS file in a vendor/plugin folder
- it should be found by bundler

Change-Id: I9edeb6b24246b7bf5373367c168db77a14a35453
Reviewed-on: https://gerrit.instructure.com/32596
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>