Add caching of dependencies for CI builds
Add caching of dependencies for CI builds

Based upon
https://blog.travis-ci.com/2013-12-05-speed-up-your-builds-cache-your-dependencies/

zulip/zulip-electron,zulip/zulip-desktop,zulip/zulip-electron,steele/zulip-electron,zulip/zulip-desktop,zulip/zulip-desktop,zulip/zulip-desktop,brainwane/zulip-electron,steele/zulip-electron,brainwane/zulip-electron,zulip/zulip-electron,steele/zulip-electron,brainwane/zulip-electron