Bump commonmarker from 0.23.6 to 0.23.8 (#57)

Author: dependabot[bot]

Gemfile.lock

@@ -14,7 +14,7 @@ GEM
       execjs
     coffee-script-source (1.11.1)
     colorator (1.1.0)
-    commonmarker (0.23.6)
+    commonmarker (0.23.8)
     concurrent-ruby (1.2.0)
     dnsruby (1.61.9)
       simpleidn (~> 0.1)