There should be single source of truth for the project version

Now the Beam version is defined in two places:

• via the 'version' parameter in gradle.properties,
• programmatically in the `apply` function in BeamModulePlugin

The latter effectively overrides the former. For someone not very familiar with the Beam build system this may result in a hard to understand and debug behavior: unexpected version gets build and/or pushed to local Maven repository, and changing version in `gradle.properties` does not have any effect.

Adding a comment with a reference to `BeamModulePlugin` to `gradle.properties` can be a good start, and in the long run it would be great to have single source of truth for the project version.