Chore: removes deprecated jsnext and main options

[gjanblaszczyk]

Description

Fixes showing deprecation warning during the building process.

rollup -c

node-resolve: setting options.jsnext is deprecated, please override options.mainFields instead
node-resolve: setting options.main is deprecated, please override options.mainFields instead

src/js/index.js → dist/video.js...

Specific Changes proposed

Uses mainFields instead of jsnext and main options.

Requirements Checklist

• [X ] Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors