Travis cache

[kuba-orlik]

This is a set of changes meant to speed up the Travis build process. It basically tells Travis to cache the node_modules directory.

1. As experiments shown, it decreases npm install time from around 30s (see the duration next to the npm install command) to around 2s
2. I've checked if adding new dependencies to package.json causes Travis to install them as well, and indeed it does
3. The overall build process time jumped from over about a minute to around 30s.

@adwydman: Please take a look, review and merge :)

[adwydman]

I reviewed and liked what I saw. :) Merging.