Resolves #188, upgrade to Asciidoctor.js 1.5.6-rc.1

[ggrossetie]

• Remove bestikk-opal-compiler dependency (we now use opal-compiler directly)
• Remove async dependency (not used anymore)
• Simplify build scripts and use ECMAScript 6 syntax
• Run the task examples on Travis
• The examples task does not run the task build anymore

[obilodeau]

One thing I'm wondering. We are providing a package-lock.json now? How and when should this be maintained?

[ggrossetie]

package-lock.json is generated by npm install and it should be commited when you upgrade, add or remove a dependency.

Basically when you edit package.json (or run npm install pkg), you should run npm install and commit both package.json and package-lock.json: https://docs.npmjs.com/files/package-lock.json

We should also probably require Node.js 6+ or 8+:

asciidoctor-reveal.js/package.json

Line 7 in 1c366b2

"node": ">=4",

I believe that package-lock.json was introduced in npm 5+ (available in Node 6+).