Change default bundle_jobs to nil to take advantage of Bundler's default behavior

[mattbrictson]

If you don't specify --jobs, Bundler will automatically choose an appropriate value based on the number of processors. We should encourage that default behavior rather than hardcoding a default of 4.

[mattbrictson]

@mauromorales I dug into Bundler's code a bit more and found that its feature to automatically set the number of jobs based on number of processors is behind a feature flag and won't be enabled until Bundler 3. What do you think the best path forward is? Do we keep the default jobs at 4 until Bundler 3 is released?

https://github.com/rubygems/bundler/blob/35be6d9a603084f719fec4f4028c18860def07f6/lib/bundler/feature_flag.rb#L33

[mattbrictson]

There is also some previous discussion here: #110

[mauromorales]

@mattbrictson I personally would still change it because then we automagically will get the new functionality in bundler once is out, and we respect upstream defaults, but I see from the previous discussion you mention (#110) that this was already decided to 4, so alternatively this issue can be put to blocked or whatever other method you use, to keep track that the change needs to happen in Bundler 3.

[mattbrictson]

Ah, I spoke too soon! The auto-jobs feature is coming soon in Bundler 2.2: https://github.com/rubygems/rubygems/releases/tag/bundler-v2.2.0.rc.1