Update appdata.xml - release 1.8.6

This commit adds release 1.8.6 (2018-08-23) in appdata.xml

Author: gvarisco

build/linux/dist/appdata.xml

@@ -36,6 +36,7 @@
   <content_rating type="oars-1.1" />
 
   <releases>
+    <release date="2018-08-23" version="1.8.6"/>
     <release date="2017-10-02" version="1.8.5"/>
     <release date="2017-08-21" version="1.8.4"/>
     <release date="2017-06-05" version="1.8.3"/>