Tick extension version. (#4975)

com.unity.ml-agents.extensions/package.json

 {
   "name": "com.unity.ml-agents.extensions",
   "displayName": "ML Agents Extensions",
-  "version": "0.0.1-preview",
+  "version": "0.1.0-preview",
   "unity": "2018.4",
   "description": "A source-only package for new features based on ML-Agents",
   "dependencies": {