build(deps): 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

@@ -129,7 +129,8 @@ GEM
     rb-fsevent (0.11.1)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.2.5)
+    rexml (3.2.8)
+      strscan (>= 3.0.9)
     rouge (3.28.0)
     safe_yaml (1.0.5)
     sass (3.7.4)
@@ -139,6 +140,7 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
     sassc (2.4.0)
       ffi (~> 1.9)
+    strscan (3.1.0)
     temple (0.8.2)
     thread_safe (0.3.6)
     tilt (2.0.10)