Enhancement: Add tests, run them on Travis

[localheinz]

This PR

• [x] requires phpunit/phpunit (and removes the minimum-stability flag)
• [x] adds a configuration for phpunit
• [x] adds coverage
• [x] adds a configuration for Travis (again)
• [x] adds a build status badge to README.md (and removes the broken badge)

:information_desk_person: This only requires to enable the build on Travis again.

[pleckey]

@localheinz I will set this up tomorrow, booked for the rest of today - it is appreciated, thank you!

[localheinz]

@pleckey

You're much welcome!

:sun_with_face: Have a nice day!

[pleckey]

@localheinz just noticed php 5.6 missing from travis run config, please add. :smile:

[localheinz]

@pleckey

Good catch!

[pleckey]

:+1: