add tests

[dbu]

there should be some tests in this repository:

• unit tests for the configuration and DI
• at least one functional test to see if the bundle can be loaded at all
• add build badge to readme: [![Build Status](https://travis-ci.org/rokka-io/rokka-client-bundle.svg?branch=master)](https://travis-ci.org/rokka-io/rokka-client-bundle)