fix(core): Delete old chart.js instance first (#375)
fix(core): Delete old chart.js instance first (#375)

Before creating new instance we need to run destroy() on old instance if it is exists to avoid **Hovering over line chart shows old chart data** bug problem, which is described here:
https://github.com/chartjs/Chart.js/issues/920
https://github.com/jtblin/angular-chart.js/issues/187



### Fix or Enhancement?
Fix

- [x] All tests passed


### Environment
- OS: Windows 7
- NPM Version: 6.1.0


apertureless/vue-chartjs,apertureless/vue-chartjs,apertureless/vue-chartjs