Bump max version to 50 (#614)

Author: stepshal

vimperator/install.rdf

@@ -17,7 +17,7 @@
             <Description>
                 <em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
                 <em:minVersion>38.0</em:minVersion>
-                <em:maxVersion>47.0</em:maxVersion>
+                <em:maxVersion>50.0</em:maxVersion>
             </Description>
         </em:targetApplication>
     </Description>