Allow browsers internal spellchecker to work

Fix a regression introduced at 731d190 that prevents in-browser spellchecker from working.
The new TinyMCE4 option is "browser_spellcheck", before it was "gecko_spellcheck".

Related ticket: http://trac.roundcube.net/ticket/1485060

program/js/editor.js

@@ -50,7 +50,8 @@ function rcube_text_editor(config, id)
       convert_urls: false, // #1486944
       image_description: false,
       paste_webkit_style: "color font-size font-family",
-      paste_data_images: true
+      paste_data_images: true,
+      browser_spellcheck: true
     };
 
   // register spellchecker for plain text editor