.travis.yml: Configure Travis to run the tests

[wking]

Docs here and here. The sed/test stuff ensures we keep 100% coverage. There is some machine-oriented coverage output in:

$ npm test -- --coverage --json --jsonOutputFile test.json

but it looks like the lines covered by each suite, when I'm more interested in the lines not covered by the entire harness.

Probably best to not merge this until we know it works ;).

Fixes #23.

[wking]

Ok, tests pass with a 100% coverage assertion now.

[jbenet]

👍 🎉