WIP: Update dependencies

Created by: mlasak

In preparation to #2880 we started to cleanup and update the dependencies (prod and dev).

Feedback very welcome!

What has been done so far and what we're still doing:

• remove outdated iron-mocha dev dep
• update mocha to a more recent version
• replace istanbul with its successor nyc
• remove outdated grunt-githooks dev dep, functionality is kept
• update babel (v6)
• remove dead grunt-jscs / jscs, introduced eslint, improve code style consistently
• remove grunt-contrib-jshint and configure eslint to do the job - [ ] update intern dev dep -> https://github.com/Dash-Industry-Forum/dash.js/issues/3370 - [ ] update imsc prod dep -> https://github.com/Dash-Industry-Forum/dash.js/issues/3371
• find replacement for jsdoc validation

What has to be done before merging

• test new dep versions on macOS
• test new dep versions on Windows
• get feedback from community
• all current functional tests / manual tests must pass