Rename retree into satellite (#84)

This commit renames the 'retree' exercise into 'satellite':

 - Decision: exercism/problem-specifications#1451
 - PR: exercism/problem-specifications#1478

Author: sshine

config.json

@@ -149,7 +149,7 @@
       "topics": []
     },
     {
-      "slug": "retree",
+      "slug": "satellite",
       "uuid": "27856806-0dee-409f-8047-357de07b914d",
       "core": false,
       "unlocked_by": null,

exercises/retree/.meta/hints.md renamed to exercises/satellite/.meta/hints.md

@@ -1,4 +1,4 @@
-# Retree
+# Satellite
 
 Imagine you need to transmit a binary tree to a satellite approaching Alpha
 Centauri and you have limited bandwidth. Since the tree has no repeating