Bump rexml from 3.3.6 to 3.3.7

Bumps [rexml](https://github.com/ruby/rexml) from 3.3.6 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@v3.3.6...v3.3.7)

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

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -163,8 +163,7 @@ GEM
     rb-inotify (0.11.1)
       ffi (~> 1.0)
     redcarpet (3.6.0)
-    rexml (3.3.6)
-      strscan
+    rexml (3.3.7)
     rouge (3.30.0)
     ruby2_keywords (0.0.5)
     sassc (2.4.0)
@@ -173,7 +172,6 @@ GEM
     slim (4.1.0)
       temple (>= 0.7.6, < 0.9)
       tilt (>= 2.0.6, < 2.1)
-    strscan (3.1.0)
     temple (0.8.2)
     thor (1.2.2)
     thread_safe (0.3.6)