Android : Keep service running after calling Activity is destroyed

I am developing an Android app that tracks the users location. I do this using a Service. The service is started using startService(..). This should keep the service running even if the app is closed (swiped from recent apps).

However, this is not the case. The service doesn't stay active, it gets restarted.

I tested this with Runtastic and RunKeeper. Both manage to keep the background service alive and continue tracking.

Here are my sources: https://bitbucket.org/alexanderglueck/biker/src/b1be133d63693fe4ef811819f23deceafaf88b8e/app/src/main/java/at/gdev/biker/?at=master

What do I need to do, to keep tracking.