Replace use of createHttpClient with mocks (#37)

package:http was recently removed from Flutter. This replaces the use of
createHttpClient, which was previously used to mock HTTP results. This
replaces that use with Mockito.

See: https://groups.google.com/d/msg/flutter-dev/AnqDqgQ6vus/spjSxiz6BgAJ

Adds 'li' style to the _buildBullet function (#34)

* added 'li' style to _buildBullet function

* updated pubspec and changelog

Declare dependencies

Rev version (#6)

Add SDK constraint