build(deps): bump net-imap from 0.4.8 to 0.4.9.1

Bumps [net-imap](https://github.com/ruby/net-imap) from 0.4.8 to 0.4.9.1.
- [Release notes](https://github.com/ruby/net-imap/releases)
- [Commits](ruby/net-imap@v0.4.8...v0.4.9.1)

---
updated-dependencies:
- dependency-name: net-imap
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Gemfile.lock

@@ -193,7 +193,7 @@ GEM
     minitest (5.20.0)
     msgpack (1.7.2)
     nested_form (0.3.2)
-    net-imap (0.4.8)
+    net-imap (0.4.9.1)
       date
       net-protocol
     net-pop (0.1.2)