Move the version check to the lib folder

Closes #4514.
Fixes #4513.

lib/devise.rb

@@ -290,6 +290,10 @@ module Devise
   mattr_accessor :token_generator
   @@token_generator = nil
 
+  def self.rails51? # :nodoc:
+    Rails.gem_version >= Gem::Version.new("5.1.x")
+  end
+
   # Default way to set up Devise. Run rails generate devise_install to create
   # a fresh initializer with all configuration values.
   def self.setup

test/rails_app/config/boot.rb

@@ -4,10 +4,6 @@ end
 
 module Devise
   # Detection for minor differences between Rails 4 and 5, and 5.1 in tests.
-  def self.rails51?
-    Rails.version.start_with? '5.1'
-  end
-
   def self.rails5?
     Rails.version.start_with? '5'
   end