Prepare release 0.8.1

README.md: typos

2.7.1 release

change versions to 2.7.0 (release)

Prepare version 2.6.0.

Prepare version 2.6.0-RC.

Prepare version 3.0.0-alpha2.

Prepare version 3.0.0-alpha1.

Prepare release 2.5.1.

build.gradle: version 2.5.0 (release), add objectboxVersionNumber and…

… objectboxVersionRelease to simplify release process