chore: Update package.json. Bump chart.js version to 2.7.3 (#433)

Update package.json.  Bump chart.js version to 2.7.3.  

We're using vue-chartjs in a web component.   Shadow DOM support was added in Version 2.7.3 chartjs/Chart.js#5585 .



### Fix or Enhancement?


- [ ] All tests passed


### Environment
- OS: Write here
- NPM Version: Write here

Author: gofreddo