Testee hangs when running testee with coverage option enabled.

For instance when we try to execute the tests from the CanJS [TodoMVC Guide](https://canjs.com/doc/guides/todomvc.html) with the command `node_modules/testee/bin/testee index.html --browsers chrome --coverage` the testee execution hangs.

After some debugging I discovered that it is required to customize babel plugins/presets in order to run `testee` with `coverage` enabled.

Also `babel-core` and `babel-plugin-istanbul` should be upgraded to the latest versions.