Travis build broken because of node/eslint version incompatibility

[cxw42]

See https://travis-ci.org/github/paradoxxxzero/gnome-shell-system-monitor-applet/builds/736886495. Node 8.12.0 is manually installed (confirmed here), but the latest eslint doesn't support node 8.x. Starting here:

$ sudo npm install -g eslint
/usr/bin/eslint -> /usr/lib/node_modules/eslint/bin/eslint.js
npm WARN notsup Unsupported engine for [email protected]: wanted: {"node":"^10.12.0 || >=12.0.0"} (current: {"node":"8.17.0","npm":"6.14.8"})
npm WARN notsup Not compatible with your version of node/npm: [email protected]

This appears to be why #635 is failing.

I see that the node 8.x was a newer version than the system's at the time it was added (commit d1b0da616bf4811cc04e33659da0756d95c30b33). However, now Travis bundles Node versions through 12.18.1 (list here). Could the manual nodejs installation simply be removed from the .travis.yml?

[DevDef]

@chrisspen Fixed in PR #660. This issue can be closed.