Specify version of Notify.

The lack of the version specification results in the following warning
on the local run:
/path/to/r8/tools/notify.py:8: PyGIWarning: Notify was imported without
specifying a version first.

Change-Id: I19a58f4d7d1e49a6fb31a388fd2c5662ae28326f
diff --git a/tools/notify.py b/tools/notify.py
index f7ae50e..f6db21b 100644
--- a/tools/notify.py
+++ b/tools/notify.py
@@ -5,12 +5,13 @@
 
 try:
   import gi
+  gi.require_version('Notify', '0.7')
   from gi.repository import Notify
-  Notify.init("R8 build tools")
+  Notify.init('R8 build tools')
 
   def notify(message):
     try:
-      Notify.Notification.new("R8 build tools", message).show()
+      Notify.Notification.new('R8 build tools', message).show()
     except:
       return