Bump net-imap from 0.5.6 to 0.5.7

Bumps [net-imap](https://github.com/ruby/net-imap) from 0.5.6 to 0.5.7.
- [Release notes](https://github.com/ruby/net-imap/releases)
- [Commits](ruby/net-imap@v0.5.6...v0.5.7)

---
updated-dependencies:
- dependency-name: net-imap
  dependency-version: 0.5.7
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -160,7 +160,7 @@ GEM
       ruby2_keywords (~> 0.0.1)
     mustermann-grape (1.1.0)
       mustermann (>= 1.0.0)
-    net-imap (0.5.6)
+    net-imap (0.5.7)
       date
       net-protocol
     net-pop (0.1.2)