Enable bundler caching for travis

[Taher-Ghaleb]

Would be interested to know why bundler caching of Travis hasn't been enabled in this repository. Thank you.

[tpitale]

🤷🏼 This rarely gets run, so I guess it never really mattered.

[Taher-Ghaleb]

I see. Thanks for your feedback, @tpitale.

[tpitale]

Happy to merge this if you think it's a good thing to have. It's been awhile since I've used Travis, or been fulltime in the ruby world. Advice is welcome. Thanks!

[Taher-Ghaleb]

@tpitale, I am currently doing research on CI caching. I am studying different aspects of it (pros and cons). It helps speed up builds, especially if there are lots of dependencies. But it may need to be maintained later to remove stale dependencies from the cache, exclude frequently changed files (e.g., installation logs). It's your choice :).

[tpitale]

Closing as Travis is going to be removed.