Bump rexml from 3.3.9 to 3.4.2 (#702)

Author: dependabot[bot]

Gemfile.lock

@@ -186,7 +186,7 @@ GEM
       mime-types (>= 1.16, < 4.0)
       netrc (~> 0.8)
     retriable (3.1.2)
-    rexml (3.3.9)
+    rexml (3.4.2)
     rouge (2.0.7)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)