fix asmdef files giving errors on 2018.3

Author: sambonfire

src/UnityExtension/Assets/Editor/GitHub.Unity/ExtensionLoader/ExtensionLoader.asmdef

@@ -1,6 +1,6 @@
 {
     "name": "ExtensionLoader",
-    "references": ["../../build/GitHub.UnityShim.dll"],
+    "precompiledReferences": ["../../build/GitHub.UnityShim.dll"],
     "includePlatforms": [
         "Editor"
     ],

src/UnityExtension/Assets/Editor/UnityTests/UnityTests.asmdef

@@ -3,8 +3,16 @@
     "references": [
         "GitHub.Unity"
     ],
+    "optionalUnityReferences": [
+        "TestAssemblies"
+    ],
     "includePlatforms": [
         "Editor"
     ],
-    "excludePlatforms": []
+    "excludePlatforms": [],
+    "allowUnsafeCode": false,
+    "overrideReferences": false,
+    "precompiledReferences": [],
+    "autoReferenced": true,
+    "defineConstraints": []
 }