Bump rexml from 3.2.6 to 3.2.8

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.6 to 3.2.8.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.6...v3.2.8)

---
updated-dependencies:
- dependency-name: rexml
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -99,7 +99,8 @@ GEM
     mini_mime (1.1.5)
     multi_xml (0.6.0)
     public_suffix (5.0.4)
-    rexml (3.2.6)
+    rexml (3.2.8)
+      strscan (>= 3.0.9)
     rspec (3.12.0)
       rspec-core (~> 3.12.0)
       rspec-expectations (~> 3.12.0)
@@ -114,6 +115,7 @@ GEM
       rspec-support (~> 3.12.0)
     rspec-support (3.12.1)
     ruby2_keywords (0.0.5)
+    strscan (3.1.0)
     webmock (3.19.1)
       addressable (>= 2.8.0)
       crack (>= 0.3.2)