Support node 6

[ranyitz]

Some projects still support node 6.

My use case is that I want to use prompts in jestjs/jest#6442

The solution for that could be fairly easy:

1. Transpile the code using babel to dist
2. Create a main file that will route between dist/index.js and lib/index.js according to the user's node version.
3. Add another travis run using node 6.

It shouldn't interfere with the development, and there will be no performance change for users that use node > 8.

Let me know your thoughts.

[terkelg]

Sounds good to me. Do you have time to make a PR? It can be combined with #66. What do you think @lukeed? I just want the simplest possible build step

[ranyitz]

@terkelg Thanks! ❤️

I've added this PR #67

The build step is simple, just run npm run build which will only need to happen on CI. If you're publishing from your machine, just make sure to build before that, so the users will get an updated dist.