Update net-imap to version 0.5.7 (#552)

Co-authored-by: depfu[bot] <23717796+depfu[bot]@users.noreply.github.com>
Co-authored-by: Joan Lledó <jlledo@redhat.com>

Author: depfu[bot]

Gemfile.lock

@@ -194,7 +194,7 @@ GEM
     multipart-post (2.1.1)
     mustermann (2.0.2)
       ruby2_keywords (~> 0.0.1)
-    net-imap (0.5.6)
+    net-imap (0.5.7)
       date
       net-protocol
     net-pop (0.1.2)