Ensure to always use the same Vue instance

I used the same technique as in vue-router https://github.com/vuejs/vue-router/blob/473b25/src/install.js#L4

Thanx Radu for the bug report!

src/index.js

@@ -14,6 +14,8 @@ var defaultConfig = {
 
 let languageVm  // Singleton.
 
+export let _Vue
+
 let GetTextPlugin = function (Vue, options = {}) {
 
   Object.keys(options).forEach(key => {
@@ -26,6 +28,8 @@ let GetTextPlugin = function (Vue, options = {}) {
     throw new Error('No translations available.')
   }
 
+  _Vue = Vue
+
   options = Object.assign(defaultConfig, options)
 
   languageVm = new Vue({

src/translate.js

@@ -1,6 +1,5 @@
-import Vue from 'vue'
-
 import plurals from './plurals'
+import { _Vue } from './index'
 
 export default {
 
@@ -14,7 +13,7 @@ export default {
   *
   * @return {String} The translated string
   */
-  getTranslation: function (msgid, n = 1, context = null, language = Vue.config.language) {
+  getTranslation: function (msgid, n = 1, context = null, language = _Vue.config.language) {
     if (!msgid) {
       return ''  // Allow empty strings.
     }
@@ -24,7 +23,8 @@ export default {
     // See the `Language` section in https://www.gnu.org/software/gettext/manual/html_node/Header-Entry.html
     // So try `ll_CC` first, or the `ll` abbreviation which can be three-letter sometimes:
     // https://www.gnu.org/software/gettext/manual/html_node/Language-Codes.html#Language-Codes
-    let translations = Vue.$translations[language] || Vue.$translations[language.split('_')[0]]
+
+    let translations = _Vue.$translations[language] || _Vue.$translations[language.split('_')[0]]
     if (!translations) {
       console.warn(`No translations found for ${language}`)
       return msgid  // Returns the untranslated string.