Update dependencies for version 15.0.0-alpha1

bower.json

   "authors": [
     "Vaadin Ltd"
   ],
-  "main": "",
+  "main": "vaadin.html",
   "license": [
     "Apache-2.0",
     "https://vaadin.com/license/cval-3.0"
     "update-core-version.js"
   ],
   "dependencies": {
-    "vaadin-core": "14.0.2",
+    "vaadin-core": "vaadin-core#15.0.0-alpha1",
     "vaadin-board": "vaadin-board#2.1.0",
     "vaadin-charts": "vaadin-charts#6.2.3",
     "vaadin-confirm-dialog": "vaadin-confirm-dialog#1.1.4",

package.json

   "author": "Vaadin Ltd",
   "license": "(Apache-2.0 OR SEE LICENSE IN https://vaadin.com/license/cval-3.0)",
   "dependencies": {
-    "@vaadin/vaadin-core": "^14.0.2",
+    "@vaadin/vaadin-core": "^15.0.0-alpha1",
     "@vaadin/vaadin-board": "^2.1.0",
     "@vaadin/vaadin-charts": "^6.2.3",
     "@vaadin/vaadin-confirm-dialog": "^1.1.4",
     "@vaadin/vaadin-grid-pro": "^2.0.3",
     "@vaadin/vaadin-rich-text-editor": "^1.0.4"
   },
-  "files": [],
+  "files": [
+    "vaadin.js"
+  ],
   "scripts": {
     "version": "node update-core-version.js && git add bower.json"
   },