chore: update to typescript 3.1

Author: Elena Hristova

demo/package.json

@@ -25,7 +25,7 @@
     "nativescript-dev-webpack": "~0.17.0",
     "tns-platform-declarations": "^5.0.0",
     "tslint": "~5.11.0",
-    "typescript": "~2.8.2"
+    "typescript": "~3.1.1"
   },
   "scripts": {
     "ci.tslint": "npm i && tslint --config '../tslint.json' 'app/**/*.ts' --exclude '**/node_modules/**'",

src/package.json

@@ -42,7 +42,7 @@
     "devDependencies": {
         "tns-core-modules": "^5.0.1",
         "tns-platform-declarations": "^5.0.0",
-        "typescript": "~2.8.2",
+        "typescript": "~3.1.1",
         "prompt": "~1.0.0",
         "rimraf": "~2.6.2",
         "tslint": "~5.11.0"