Bump rexml from 3.3.1 to 3.3.7

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.1 to 3.3.7.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.3.1...v3.3.7)

---
updated-dependencies:
- dependency-name: rexml
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Gemfile.lock

@@ -78,8 +78,7 @@ GEM
     rainbow (3.1.1)
     rake (13.1.0)
     regexp_parser (2.9.2)
-    rexml (3.3.1)
-      strscan
+    rexml (3.3.7)
     rspec (3.12.0)
       rspec-core (~> 3.12.0)
       rspec-expectations (~> 3.12.0)
@@ -134,7 +133,6 @@ GEM
       simplecov
     simplecov-lcov (0.8.0)
     simplecov_json_formatter (0.1.4)
-    strscan (3.1.0)
     thor (1.3.0)
     tzinfo (2.0.6)
       concurrent-ruby (~> 1.0)