Make notify-send notifications transient

In contrast to Ubuntu Shell in Gnome Shell notifications are persistent by default. This change adds a hint for notify-send to make notifications emitted by gradle transient. Without that notifications clutter the notification panel in Gnome Shell.

![Workspace 1_023](https://f.cloud.github.com/assets/205451/43794/f7d1f842-5698-11e2-946e-2523fa7c4edc.png)