Consider npm audit checks / autofixes to be added to CI

[C3-TKO]

Is your feature request related to a problem? Please describe. This project contains a lot of outdated dependencies. Some of them contain even critical security risks.

found 796 vulnerabilities (64 low, 3 moderate, 727 high, 2 critical) in 47508 scanned packages
  run `npm audit fix` to fix 732 of them.
  64 vulnerabilities require semver-major dependency updates.

Describe the solution you'd like Consider adding a non failing audit stage to the CI process in Travis. At the beginning it should sufficient to just run npm run audit or even npm run audit --fix just to be aware of the outdated issues.

Describe alternatives you've considered If coverage of this project is high enough you could even consider to incorporate services like https://greenkeeper.io to keep dependencies up-to-date automatically

[sgobotta]

Merged in #207