[rubygems/rubygems] Update GitLab CI template with new one

GitLab CI now needs the default keyword on specification of image
and before_script.

https://docs.gitlab.com/ee/ci/yaml/#default

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

https://github.com/rubygems/rubygems/commit/b79e78e733

lib/bundler/templates/newgem/gitlab-ci.yml.tt

@@ -1,8 +1,9 @@
-image: ruby:<%= RUBY_VERSION %>
+default:
+  image: ruby:<%= RUBY_VERSION %>
 
-before_script:
-  - gem install bundler -v <%= Bundler::VERSION %>
-  - bundle install
+  before_script:
+    - gem install bundler -v <%= Bundler::VERSION %>
+    - bundle install
 
 example_job:
   script: