Update documentation for karma & jasmine versions

Turns out Ionic3 is not compatible with the most recent Karma and Jasmine versions since the former runs on webpack 3 and the latter two require webpack 4. This is a blocking incompatibility and ionic apps fail building : 

https://forum.ionicframework.com/t/main-js-vendor-js-gone-after-karma-jasmine-installation-and-webpack-update/150525