Revert changes to ShaderGraph version

com.unity.render-pipelines.high-definition/package.json

     "dependencies": {
         "com.unity.postprocessing": "2.0.7-preview",
         "com.unity.render-pipelines.core": "3.0.0-preview",
-        "com.unity.shadergraph": "4.0.0-preview"
+        "com.unity.shadergraph": "3.0.0-preview"
     }
 }

com.unity.render-pipelines.lightweight/package.json

     "dependencies": {
         "com.unity.postprocessing": "2.0.7-preview",
         "com.unity.render-pipelines.core": "3.0.0-preview",
-        "com.unity.shadergraph": "4.0.0-preview"
+        "com.unity.shadergraph": "3.0.0-preview"
     }
 }

com.unity.shadergraph/package.json

 {
     "name": "com.unity.shadergraph",
     "description": "Shader Graph",
-    "version": "4.0.0-preview",
+    "version": "3.0.0-preview",
     "unity": "2018.2",
     "displayName": "Shader Graph",
     "dependencies": {