refactor tests and babel config. Fixes target browsers.

This is basically the change propagated from the current
default preact tempalate. Now the code compiles according to set
browsers.

tests/__mocks__/fileMocks.js

@@ -0,0 +1,3 @@
+// This fixed an error related to the CSS and loading gif breaking my Jest test
+// See https://facebook.github.io/jest/docs/en/webpack.html#handling-static-assets
+module.exports = 'test-file-stub';