Bump mail from 2.7.1 to 2.8.1

Bumps [mail](https://github.com/mikel/mail) from 2.7.1 to 2.8.1.
- [Changelog](https://github.com/mikel/mail/blob/2.8.1/CHANGELOG.rdoc)
- [Commits](mikel/mail@2.7.1...2.8.1)

---
updated-dependencies:
- dependency-name: mail
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Gemfile.lock

@@ -193,8 +193,11 @@ GEM
     loofah (2.21.3)
       crass (~> 1.0.2)
       nokogiri (>= 1.12.0)
-    mail (2.7.1)
+    mail (2.8.1)
       mini_mime (>= 0.1.1)
+      net-imap
+      net-pop
+      net-smtp
     maildown (3.3.1)
       actionmailer (>= 4.0.0)
       kramdown-parser-gfm
@@ -206,7 +209,7 @@ GEM
       mime-types-data (~> 3.2015)
     mime-types-data (3.2019.1009)
     mini_histogram (0.3.1)
-    mini_mime (1.1.2)
+    mini_mime (1.1.5)
     mini_portile2 (2.8.2)
     minitest (5.18.1)
     mocha (1.16.0)