Fix uber-JAR build

[ctrueden]

The build uses maven-assembly-plugin to produce a with-libs classifier artifact that includes slim-curve native libs for the six major platforms (Windows, macOS, Linux; 32-bit and 64-bit). Unfortunately, the current master branch does not properly aggregate these six artifacts. It needs to be fixed, so that Travis deploys the right SNAPSHOT, so that the slim-plugin component works properly.