Upgrade: Bump rexml from 3.2.5 to 3.2.8

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.5 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@v3.2.5...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
     rb-inotify (0.10.1)
       ffi (~> 1.0)
     redcarpet (3.6.0)
-    rexml (3.2.5)
+    rexml (3.2.8)
+      strscan (>= 3.0.9)
     rouge (3.30.0)
     sass (3.7.4)
       sass-listen (~> 4.0.0)
@@ -112,6 +113,7 @@ GEM
     sprockets (3.7.2)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
+    strscan (3.1.0)
     temple (0.10.0)
     thor (1.2.1)
     tilt (2.0.11)