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:
chartjs/Chart.js#920
jtblin/angular-chart.js#187



### Fix or Enhancement?
Fix

- [x] All tests passed


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

Author: payalord

src/BaseCharts.js

@@ -63,6 +63,7 @@ export function generateChart (chartId, chartType) {
         this.$data._plugins.push(plugin)
       },
       renderChart (data, options) {
+        if (this.$data._chart) this.$data._chart.destroy()
         this.$data._chart = new Chart(
           this.$refs.canvas.getContext('2d'), {
             type: chartType,