Do not use 'this' in translate.js

'this' is not defined when not installing the plugin on Vue.

Fix Polyconseil#113 (comment)

Author: janlazo

src/translate.js

@@ -11,7 +11,7 @@ let _config = {
 }
 let _translations = {}
 
-export default {
+const translate = {
 
   /*
    * Get the translated string from the translation.json file generated by easygettext.
@@ -118,7 +118,7 @@ export default {
    * @return {String} The translated string
   */
   'gettext': function (msgid) {
-    return this.getTranslation(msgid)
+    return translate.getTranslation(msgid)
   },
 
   /*
@@ -131,7 +131,7 @@ export default {
    * @return {String} The translated string
   */
   'pgettext': function (context, msgid) {
-    return this.getTranslation(msgid, 1, context)
+    return translate.getTranslation(msgid, 1, context)
   },
 
   /*
@@ -146,7 +146,7 @@ export default {
    * @return {String} The translated string
   */
   'ngettext': function (msgid, plural, n) {
-    return this.getTranslation(msgid, n, null, plural)
+    return translate.getTranslation(msgid, n, null, plural)
   },
 
   /*
@@ -162,7 +162,7 @@ export default {
    * @return {String} The translated string
   */
   'npgettext': function (context, msgid, plural, n) {
-    return this.getTranslation(msgid, n, context, plural)
+    return translate.getTranslation(msgid, n, context, plural)
   },
 
   /*
@@ -183,3 +183,5 @@ export default {
   },
 
 }
+
+export default translate