[models] Set default manifest version to 17

This is what EGS currently delivers, and it also
appears there's a shorter header for some lower
versions so for compatibilities' sake just use 17.

legendary/models/manifest.py

@@ -69,7 +69,7 @@ class Manifest:
         self.size_uncompressed = 0
         self.sha_hash = ''
         self.stored_as = 0
-        self.version = 0
+        self.version = 17
         self.data = b''
 
         # remainder