Update Mynewt repository.yaml for 1.9 release

Missed this update for the release.  This should point Mynewt to the
latest release.
Signed-off-by: David Brown <david.brown@linaro.org>

repository.yml

@@ -34,9 +34,10 @@ repo.versions:
     "1.7.1": "v1.7.1"
     "1.7.2": "v1.7.2"
     "1.8.0": "v1.8.0"
+    "1.9.0": "v1.9.0"
 
     "0-dev": "0.0.0"        # main
-    "0-latest": "1.8.0"     # latest stable release
-    "1-latest": "1.8.0"     # latest stable release
+    "0-latest": "1.9.0"     # latest stable release
+    "1-latest": "1.9.0"     # latest stable release
 
-    "1.0-latest": "1.8.0"
+    "1.0-latest": "1.9.0"