Update net-imap to version 0.5.6

Author: depfu[bot]

Gemfile.lock

@@ -204,7 +204,7 @@ GEM
       uri
     net-http-persistent (4.0.5)
       connection_pool (~> 2.2)
-    net-imap (0.5.5)
+    net-imap (0.5.6)
       date
       net-protocol
     net-pop (0.1.2)