Improve Travis CI build Performance

[chenzhang22]

Caching Dependencies and Directories Travis CI can cache content that does not often change, to speed up the build process.

===================== If there are any inappropriate modifications in this PR, please give me a reply and I will change them.

[peterdemaeyer]

AFAICT Travis CI builds no longer work on this repo since somewhere in May, because Travis has moved from .org to .com. The owner of this repo should link it to their account on the new https://app.travis-ci.com.