Try using activesupport 4.2.7

This is the latest release prior to activesupport 5.0.0
which requires Ruby 2.2.2 or later.

We're still using Ruby 2.1.7 as used by GitHub Pages, see
https://pages.github.com/versions/ which does not say
which version of activesupport is used.

Author: peterjc

Gemfile

@@ -1,3 +1,4 @@
 source 'https://rubygems.org'
+gem 'activesupport', '4.2.7'
 gem 'github-pages'
 gem 'jekyll-redirect-from'