fix: lock connection_pool to v2 when using react-rails (#634)

Currently `react-rails` does not work with `connection_pool` v3, so we
need to lock to v2 until
reactjs/react-rails#1367 is released

Author: G-Rath

variants/frontend-react/template.rb

@@ -3,6 +3,7 @@
 TERMINAL.puts_header "Adding react-rails to Gemfile"
 
 gem "react-rails"
+gem "connection_pool", "~> 2.0"
 run "bundle install"
 
 run "rails generate react:install"