Require Node.js 10

[antleblanc]

Drop support of Node.js 6 and 8.

Following the Breaking Changes introduced in the Rollup v2.0.0, I'm proposing to update the .travis.yml configuration to be aligned wit the Node.js versions supported.

Breaking Changes:

• Rollup now requires at least Node 10 to run(…)

Source: https://github.com/rollup/rollup/releases/tag/v2.0.0

Regarding how .travis.yml can be configured, I decided to list all supported versions instead of using lts/*.

Sources:

• https://docs.travis-ci.com/user/languages/javascript-with-nodejs/#specifying-nodejs-versions
• https://nodejs.org/en/about/releases/

This Pull Request can be merged right after #62.

Signed-off-by: Antoine Leblanc ant.leblanc@gmail.com