Enable dependency caching for travis

[Taher-Ghaleb]

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

[stolstov]

Thank you for the PR. It is not obvious to me, what kind of issue this is causing and what you are trying to resolve. Could you clarify?

[Taher-Ghaleb]

Hi @stolstov. This helps speed up builds by caching maven dependencies onto the Travis CI server. So, instead of installing dependencies for every build, Travis will use the binaries of dependencies from the cache. Hope that clarifies the purpose of this PR. You may read more about it here. Thanks.

[stolstov]

@Taher-Ghaleb Thank you for the explanation. According to the build history on travis, the builds are a little over 4 minutes. I've added a lot of code here and never had a thought about travis build being an issue.

Out of curiosity do you have any numbers to that show things are significantly faster with your change?

[Taher-Ghaleb]

I see. It's because that this repository has only a few dependencies. Reduction of build speed could be significant when the repository has so many dependencies. I am still researching this on various repositories to explore how the improvement of caching differs from one repo to another. Thank you.