Add `files` to `package.json`.

[wtgtybhertgeghgtwtg]

Prevents .travis.yml and test.js from being published to NPM.

[watson]

@wtgtybhertgeghgtwtg Though it would be nice to be able to limit this, I'm really not keen on using the files property, as I don't know of any way to detect if I remember to keep that field up to date. The issue is that my tests will have all files available, so they will not discover when a file isn't added, but I might publish broken packages to npm. Let me know if you have any good solution to this and I'll happily reevaluate this PR 😃