fix: Gemfile & Gemfile.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-REXML-8309365

Author: snyk-bot

Gemfile

@@ -10,4 +10,4 @@ gem 'html-proofer', '~> 5.0', '>= 5.0.9'
 gem "jekyll-redirect-from", ">= 0.16.0"
 gem "kramdown-parser-gfm", ">= 1.1.0"
 gem "jekyll-sitemap", ">= 1.4.0"
-gem 'rexml', '~> 3.3', '>= 3.3.8'
+gem 'rexml', '~> 3.3', '>= 3.3.9'

Gemfile.lock

@@ -105,7 +105,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.3.8)
+    rexml (3.3.9)
     rouge (4.2.1)
     ruby-rc4 (0.1.5)
     safe_yaml (1.0.5)
@@ -140,7 +140,7 @@ DEPENDENCIES
   jekyll-sitemap (>= 1.4.0)
   kramdown-parser-gfm (>= 1.1.0)
   nokogiri (~> 1.16)
-  rexml (~> 3.3, >= 3.3.8)
+  rexml (~> 3.3, >= 3.3.9)
 
 BUNDLED WITH
    2.4.2