Check for Rails.env instead of Rails

`Rails` constant is added by rails-html-sanitizer leading to bugs in
non-Rails apps using ActiveRecord and ActionMailer

To reproduce:
```
unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'action_mailer'
require 'minitest/autorun'
require 'logger'

class TestMailer < ActionMailer::Base
end


class BugTest < Minitest::Test
  def test_establish_connection
    ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:') # => NoMethodError: undefined method `env' for Rails:Module
  end
end
```