Update composer.json file with Net_Sieve package requirement

plugins/managesieve/composer.json

@@ -15,10 +15,15 @@
         {
             "type": "composer",
             "url": "http://plugins.roundcube.net"
+        },
+        {
+            "type": "pear",
+            "url": "http://pear.php.net/"
         }
     ],
     "require": {
         "php": ">=5.3.0",
-        "roundcube/plugin-installer": ">=0.1.3"
+        "roundcube/plugin-installer": ">=0.1.3",
+        "pear-pear/Net_Sieve": ">=1.3.2"
     }
 }