findIndex is not working on IE11

[mlumbroso]

Hello,

the component triggers an issue with IE11, because of the use of findIndex in components/VueTabs.js
There are a few workarounds : https://stackoverflow.com/questions/37698996/findindex-method-issue-with-internet-explorer

Can you fix this ? I can make a PR otherwise !
Thanks :)

Edit : also noticed 3 "+" signs here :

vue-tabs/src/components/VueTabs.js

Line 107 in 4df45c3

+ if (indexToActivate === this.activeTabIndex && !this.tabs[this.activeTabIndex].active) {

[mlumbroso]

@cristijora any news on this one ?
Thanks