Bump rexml from 3.3.2 to 3.3.7

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.2 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@v3.3.2...v3.3.7)

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

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

Gemfile.lock

@@ -15,15 +15,13 @@ GEM
       base64 (>= 0.1.0)
       rack (~> 2.2, >= 2.2.4)
     rack-ssl-enforcer (0.2.9)
-    rexml (3.3.2)
-      strscan
+    rexml (3.3.7)
     ruby2_keywords (0.0.5)
     sinatra (3.2.0)
       mustermann (~> 3.0)
       rack (~> 2.2, >= 2.2.4)
       rack-protection (= 3.2.0)
       tilt (~> 2.0)
-    strscan (3.1.0)
     thin (1.8.2)
       daemons (~> 1.0, >= 1.0.9)
       eventmachine (~> 1.0, >= 1.0.4)