Remove extraneous installation of bundler

.gitlab/ci/rails.gitlab-ci.yml

@@ -9,7 +9,7 @@
 .base-script: &base-script
   # Only install knapsack after bundle install! Otherwise oddly some native
   # gems could not be found under some circumstance. No idea why, hours wasted.
-  - run_timed_command "gem install bundler:1.17.3 knapsack --no-document"
+  - run_timed_command "gem install knapsack --no-document"
   - run_timed_command "scripts/gitaly-test-build"
   - run_timed_command "scripts/gitaly-test-spawn"
   - source ./scripts/rspec_helpers.sh