Drop support for Rails 3.2

**Why**: To be able to support Rails 5 without deprecation warnings,
we need to replace `before_filter` with `before_action`.
`before_action` is not supported in Rails 3.2, so we need to bump the
major version number since this will be a breaking change for people
who can't upgrade Rails.

.travis.yml

@@ -1,7 +1,6 @@
 language: ruby
 
 env:
-  - "RAILS_VERSION=3.2"
   - "RAILS_VERSION=4.0"
   - "RAILS_VERSION=4.1"
   - "RAILS_VERSION=4.2"

app/controllers/devise/two_factor_authentication_controller.rb

@@ -1,11 +1,6 @@
 class Devise::TwoFactorAuthenticationController < DeviseController
-  if Rails::VERSION::MAJOR >= 4
   prepend_before_action :authenticate_scope!
   before_action :prepare_and_validate, :handle_two_factor_authentication
-  else
-    prepend_before_filter :authenticate_scope!
-    before_filter :prepare_and_validate, :handle_two_factor_authentication
-  end
 
   def show
   end

lib/two_factor_authentication/controllers/helpers.rb

@@ -4,11 +4,7 @@ module TwoFactorAuthentication
       extend ActiveSupport::Concern
 
       included do
-        if Rails::VERSION::MAJOR >= 4
         before_action :handle_two_factor_authentication
-        else
-          before_filter :handle_two_factor_authentication
-        end
       end
 
       private

spec/rails_app/app/controllers/home_controller.rb

@@ -1,9 +1,5 @@
 class HomeController < ApplicationController
-  if Rails::VERSION::MAJOR >= 4
   before_action :authenticate_user!, only: :dashboard
-  else
-    before_filter :authenticate_user!, only: :dashboard
-  end
 
   def index
   end