dependencies: pin activesupport to v4.2.7

Temporary fix until these are fixed:

- jekyll/jemoji#46
- jekyll/jekyll-mentions#36

lib/github-pages/dependencies.rb

@@ -31,6 +31,9 @@ class Dependencies
       # Pin listen because it's broken on 2.1 & that's what we recommend.
       # https://github.com/guard/listen/pull/371
       "listen"                    => "3.0.6"
+
+      # Pin activesupport because 5.0 is broken on 2.1
+      "activesupport"             => "4.2.7"
     }.freeze
 
     # Jekyll and related dependency versions as used by GitHub Pages.