CI testing fixes

[brodycj]

• Drop deprecated Node.js 4 support from CI testing (originally proposed in PR #215)
• Add Node.js 10 to CI testing (originally proposed in PR #217)
• .travis.yml add vertical spacing & newline at the end (originally proposed in PR #216)

I think we should also increase version to 2.0.0-dev when we drop Node.js 4 if we want to follow semver on this tool.

This proposal was originally motivated by the spurious AppVeyor failures on Node.js 4 observed in PR #213.

A quick review would be appreciated, would like to merge in a squash commit if there are no major objections.

[brodycj]

Thanks @janpio!