fix(plugin-vue): Access the window object safely, in case it isn't defined

[imjoehaines]

This prevents errors when using SSR as window won't exist

The tests for this aren't particularly nice as we have to edit the global object to add/remove references to window and window.Vue; I'm not sure there's a nicer way to write them ?

Fixes #895

[bugsnagbot]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	39.56 kB	12.22 kB
After	39.56 kB	12.22 kB
±	No change	No change

Generated by 🚫 dangerJS against aa887ea

[bengourley]

Agreed that tests seem pretty nasty, but I can't think of a better way to do them.