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

@@ -77,7 +77,8 @@ GEM
     regexp_parser (2.8.3)
     reline (0.4.1)
       io-console (~> 0.5)
-    rexml (3.2.6)
+    rexml (3.2.8)
+      strscan (>= 3.0.9)
     rouge (3.30.0)
     rubocop (1.42.0)
       json (~> 2.3)
@@ -101,6 +102,7 @@ GEM
     sassc (2.4.0)
       ffi (~> 1.9)
     stringio (3.1.0)
+    strscan (3.1.0)
     terminal-table (2.0.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     test-unit (3.6.1)