Fix #46

Fix the issue 46 - https://github.com/github/maven-plugins/issues/46

* migrate to the Maven-API 3.1.0
* remote Aether dependency - unfortunately, the code using Aether cannot work anymore. username and password cannot be retrieved from the Authentication object anymore. We still rely on the settings.xml
* update github-core version in the download plugin

Signed-off-by: Clement Escoffier <clement.escoffier@gmail.com>