Python 2.6 and tk-core 0.15 fixes

tk-core v0.18.99: Ensures set_authenticated_user is available before
calling it. This won't be the case if you are jumping into a 0.15-based
project.

tk-desktop v2.3.7: Fixes a crash when selecting a project using Python
2.6.

Author: mathurf

core/core_api.yml

@@ -14,4 +14,4 @@
 location:
   type: app_store
   name: tk-core
-  version: v0.18.97
+  version: v0.18.99

env/includes/common/engines.yml

@@ -51,7 +51,7 @@ common.engines.tk-flame.location:
 common.engines.tk-desktop.location:
     type: app_store
     name: tk-desktop
-    version: v2.3.6
+    version: v2.3.7
 
 # Shell
 common.engines.tk-shell.location: