chore(deps-dev): bump rexml from 3.2.8 to 3.3.9 in /api

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.8 to 3.3.9.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.8...v3.3.9)

---
updated-dependencies:
- dependency-name: rexml
  dependency-type: indirect
...

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

api/Gemfile.lock

@@ -242,8 +242,7 @@ GEM
       railties (>= 5.2)
     reverse_markdown (2.1.1)
       nokogiri
-    rexml (3.2.8)
-      strscan (>= 3.0.9)
+    rexml (3.3.9)
     rspec-core (3.13.0)
       rspec-support (~> 3.13.0)
     rspec-expectations (3.13.1)
@@ -301,7 +300,6 @@ GEM
       listen (>= 2.7, < 4.0)
       spring (>= 4)
     stringio (3.1.1)
-    strscan (3.1.0)
     thor (1.3.1)
     tilt (2.3.0)
     timeout (0.4.1)