Re-format .travis.yml

[campionfellin]

Adds node_modules to cache, so it won't reinstall unless there are new ones to add.
Removes node versions < 6
Installs and builds before linting/testing
After testing runs coverage report

Signed-off-by: campionfellin campionfellin@gmail.com

[campionfellin]

Looks like build failed for linux because build starts with rm index.js. Is there a reason we do this? Doesn't it just overwrite it when we run tsc --project tsconfig.json anyway?

[grant]

Yes. We should remove rm index.js, as it will always fail.
I did it more for debugging purposes at the time.