Deprecate node version 8 to fix travis test failures

[sashadev-sky]

Fixes #0000 (<=== Add issue number here)

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=========================

Summary

• Deprecate node version 8 (many of our dev dependencies deprecate it in their newer versions)

  • Support node versions 10, and 12 (12.16.2 is the LTS) right now.

• Update packages that have a also deprecated node 8 in newer versions

• Remove deprecated jshint attributes from Gruntfile.js

• bump to 0.16.6

• add below field to give warning to users:

"engines": {
    "node": ">=10.0.0"
  },

[sashadev-sky]

@jywarren @VladimirMikulic our travis tests have been pretty fickle with failures (see for ex. last commit to master https://travis-ci.org/github/publiclab/Leaflet.DistortableImage/jobs/675056848)
so i'm updating the node versions from 8, 10, and 12 -> 10 and 12. (8 is deprecated by karma, sinon, cross-env and 10 is the lowest active LTS).

[jywarren]

Hi, thank you! I also found some more issues related to this which I've documented in comments in #675, check it out!