Enable depedency caching for travis

[Taher-Ghaleb]

Would be interested to know why maven dependencies haven't been cached on Travis. Thank you.

[findepi]

Thanks for your PR.

Caching introduces complexity. For example you would need to clear the cache from your own snapshot builds to make sure CI runs in isolation.

Also, we plan to migrate to Github Actions, so we do plan to improve our travis configuration.
Hope you understand.

[Taher-Ghaleb]

I got your point. Thanks for your feedback.

[dain]

This project has migraged to Github actions