Update all dependencies, fresh Node.js versions at Travis, and fix one typo in test

These dependencies were updated:

 autoprefixer            ^8.2.0  →  ^10.4.0
 babel-plugin-istanbul   ^5.1.4  →   ^6.1.1
 clean-css              ^4.1.11  →   ^5.2.2
 coveralls               ^3.0.0  →   ^3.1.1
 cross-env               ^5.1.4  →   ^7.0.3
 cssnano                ^4.1.10  →   ^5.0.8
 eslint                 ^4.18.2  →   ^8.1.0
 mocha                   ^5.0.5  →   ^9.1.3
 node-sass              ^4.12.0  →   ^6.0.1
 npm-run-all             ^4.1.1  →   ^4.1.5
 nyc                    ^14.1.1  →  ^15.1.0
 postcss                ^6.0.21  →  ^8.3.11
 postcss-nested          ^2.1.2  →   ^5.0.6
 precss                  ^3.1.2  →   ^4.0.0
 rollup                 ^0.50.0  →  ^2.58.3
 rollup-plugin-eslint    ^6.0.0  →   ^7.0.0
 rollup-plugin-postcss   ^2.0.3  →   ^4.0.1
 rollup-plugin-uglify    ^2.0.1  →   ^6.0.4

Before update, npm install reported:

found 141 vulnerabilities (1 low, 116 moderate, 21 high, 3 critical)

After update, npm install reported:

found 28 vulnerabilities (1 low, 16 moderate, 10 high, 1 critical)

Since Travis CI changed their pricing model some time ago, it might be more efficient to run the automated tests at GitHub Actions.

frappe / charts

Update all dependencies, fresh Node.js versions at Travis, and fix one typo in test #363