Revert "Specify specific version of bundler in .drone.yml"

This reverts commit 2d358b47b7.
I don't think it's relevant after the Gemfile.lock update?

.drone.yml

@@ -5,7 +5,7 @@ steps:
   - name: build
     image: ruby
     commands:
-      - gem install bundler:1.17.3
+      - gem install bundler
       - bundle install
       - bundle exec jekyll build