Chart.js build is fragile and depend on potentially unstable external code

During the build of the webjar module, https://raw.githubusercontent.com/google/palette.js/master/palette.js is downloaded (see https://github.com/xwiki-contrib/application-chartjs/blob/21fc551d1b40a8f04ef2a1d774968400df1f391b/application-chartjs-webjar/pom.xml#L60).

We have no guarantee that this file won't change to something unstable/incompatible (even though the project seems dead), and break the extension.

Note: the affect version needs to be refined