Gemfile.rb

# this don't work (bundler don't support nested platform/grouping https://github.com/carlhuda/bundler/issues/1298)
group :development, :test do
    gem 'pg', :platforms => :ruby
    gem 'activerecord-jdbcpostgresql-adapter', :platforms => :jruby
end

# you should do something like this
group :development, :test do
    gem 'pg' if RUBY_PLATFORM != 'java'
    gem 'activerecord-jdbcpostgresql-adapter' if RUBY_PLATFORM == 'java'
end

onde está o seu PR consertando essa parada? :-)