update the way of embedding Chart.js

[hongbo-miao]

https://github.com/chartjs/Chart.js/releases, check Version 2.2.0 part:

Starting with v2.2.0-rc.1 Chart.js uses a new build system. We no longer include the checked in files (dist folder) in the repository.

[simonbrunel]

@hongbo-miao @valorkin this PR introduced a wrong way to include Chart.js and should be reverted.

The release notes actually said:

Starting with v2.2.0-rc.1 Chart.js uses a new build system. We no longer include the checked in files (dist folder) in the repository. When a tagged release is made, a build will occur and will be published on NPM automatically.

Which mean that the dist files was (and still are) in the npm package under the dist folder.

It seems that this change generated lot of confusion for ng2-charts users because it doesn't work to import src/chart.js via a script tag (#526, #679, #832, #969, #1041, etc.). Also, starting 2.8, the src will be removed from the npm package (chartjs/Chart.js#6105): we consider these files private and our users shouldn't rely on it.