Cache dependencies in Travis

[tirkarthi]

This PR caches Travis deps. This reduces the load on Maven and Clojars. It also makes the builds faster. Time on my module builds as below :

• Before cache - 55 seconds
• After cache - 44 seconds (No dependencies downloaded from Maven and Clojars)

Ref : https://docs.travis-ci.com/user/caching/#Arbitrary-directories

[vedang]

Thanks, merged!