Fix bundler in CI

[Bundler 2.5 dropped support for Ruby 2.7](https://github.com/rubygems/rubygems/releases/tag/bundler-v2.5.0).
Since we are testing with Ruby 2.7, we should pin the version of Bundler
to 2.4.x since this is the last version that's compatible with 2.7.

.github/workflows/tests.yml

@@ -45,7 +45,7 @@ jobs:
       run: |
         sudo apt-get -yqq install libpq-dev postgresql-client
         createdb que-test
-        gem install bundler
+        gem install bundler --version '~> 2.4.0'
         bundle install --jobs 4 --retry 3
         USE_RAILS=true bundle exec rake test
         bundle exec rake test