Bump net-imap from 0.3.7 to 0.4.0

Bumps [net-imap](https://github.com/ruby/net-imap) from 0.3.7 to 0.4.0.
- [Release notes](https://github.com/ruby/net-imap/releases)
- [Commits](ruby/net-imap@v0.3.7...v0.4.0)

---
updated-dependencies:
- dependency-name: net-imap
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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

Gemfile.lock

@@ -152,7 +152,7 @@ GEM
     mocha (2.1.0)
       ruby2_keywords (>= 0.0.5)
     msgpack (1.7.2)
-    net-imap (0.3.7)
+    net-imap (0.4.0)
       date
       net-protocol
     net-pop (0.1.2)