[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.

Prepare spec for bulk enqueue refactor to support bulk enqueue of jobs
with differing job class and job options.

Allow bulk enqueue of multiple different job classes and differing job
options in a single `.bulk_enqueue` block. Each job can now differ by
job class, queue, priority, run at and tags (in addition to args and
kwargs).