force name of zip dist to activator-${version}

sbt-native-packager defaults to activator-dist-${version} if we
don't set it since it uses the project name and we want 'activator'

And the way we set it before with <<= resulted in sbt-native-packager
building on our name with <<= and adding the version another time.
Merge pull request #63 from typesafehub/wip/havocp-fix-dist-name

force name of zip dist to activator-${version}