Bump rexml from 3.2.6 to 3.3.3 in /rblib

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.6 to 3.3.3.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.6...v3.3.3)

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

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

Author: dependabot[bot]

rblib/Gemfile.lock

@@ -13,7 +13,8 @@ GEM
     power_assert (2.0.3)
     public_suffix (5.0.3)
     rake (13.0.6)
-    rexml (3.2.6)
+    rexml (3.3.3)
+      strscan
     rspec (3.12.0)
       rspec-core (~> 3.12.0)
       rspec-expectations (~> 3.12.0)
@@ -28,6 +29,7 @@ GEM
       rspec-support (~> 3.12.0)
     rspec-support (3.12.1)
     ruby2_keywords (0.0.5)
+    strscan (3.1.0)
     test-unit (3.6.1)
       power_assert
     unidecoder (1.1.2)