Remove Vue runtime dependency

This allow the bundle to be used in jsfiddle, plnkr, jsbin, etc.,
as the bundle does not depend on Vue.

The problem lies on second line of bundle

```js
"object" == typeof exports ? exports["vue-js-modal"] = factory(require("vue")) : root["vue-js-modal"] = factory(root.vue)
```

where `factory` tries to find `root.vue`, but the official Vue bundle exports
as `root.Vue`.

Removing the runtime dependency solves this.

`Object.assign` is available in all browsers but IE:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign

src/Modal.vue

@@ -28,7 +28,6 @@
   </transition>
 </template>
 <script>
-  import Vue         from 'vue'
   import Modal       from './index'
   import Resizer     from './Resizer.vue'
   import { inRange } from './util'
@@ -348,7 +347,7 @@
           ref: this.$refs.modal
         }
 
-        return Vue.util.extend(data, params || {})
+        return Object.assign(data, params || {})
       },
       /**
        * Event handler which is triggered on modal resize