* Further improve Gradle build, update installation instructions

* update installation and building instructions in `README.md` to include new tasks (like `runJosm`)
* update versions of Gradle plugins `nexus-staging` and `spotless`, remove `download` plugin
* automate activation of the plugin and starting of JOSM when running `installPlugin`
* move configuration of JOSM and its manifest to `build.gradle` (if there's no Ant build, that's the preferred method)
* distribute the Atlas dependency inside the release *.jar

* Let Travis CI also check, if compilation against the minimum compatible JOSM version is possible

Also put the version back to `gradle.properties`, because `build.sh` depends on it.