Introduce `Rails.gem_version`

This method return `Gem::Version.new(Rails.version)`, suggesting a more
reliable way to perform version comparison.

Example:

    Rails.version #=> "4.1.2"
    Rails.gem_version #=> #<Gem::Version "4.1.2">

    Rails.version > "4.1.10" #=> false
    Rails.gem_version > Gem::Version.new("4.1.10") #=> true
    Gem::Requirement.new("~> 4.1.2") =~ Rails.gem_version #=> true

This was originally introduced as `.version` by @charliesome in #8501
but got reverted in #10002 since it was not backward compatible.

Also, updating template for `rake update_versions`.

2 files changed, 19 insertions, 7 deletions

diff --git a/activemodel/lib/active_model/gem_version.rb b/activemodel/lib/active_model/gem_version.rb
new file mode 100644
index 0000000000..964b24398d
--- /dev/null
+++ b/activemodel/lib/active_model/gem_version.rb
@@ -0,0 +1,15 @@
+module ActiveModel
+  # Returns the version of the currently loaded ActiveModel as a <tt>Gem::Version</tt>
+  def self.gem_version
+    Gem::Version.new VERSION::STRING
+  end
+
+  module VERSION
+    MAJOR = 4
+    MINOR = 2
+    TINY  = 0
+    PRE   = "alpha"
+
+    STRING = [MAJOR, MINOR, TINY, PRE].compact.join(".")
+  end
+end
diff --git a/activemodel/lib/active_model/version.rb b/activemodel/lib/active_model/version.rb
index 5c9d2bc6bc..b1f9082ea7 100644
--- a/activemodel/lib/active_model/version.rb
+++ b/activemodel/lib/active_model/version.rb
@@ -1,11 +1,8 @@
+require_relative 'gem_version'
+
 module ActiveModel
-  # Returns the version of the currently loaded ActiveModel as a Gem::Version
+  # Returns the version of the currently loaded ActiveModel as a <tt>Gem::Version</tt>
   def self.version
-    Gem::Version.new "4.2.0.alpha"
-  end
-
-  module VERSION #:nodoc:
-    MAJOR, MINOR, TINY, PRE = ActiveModel.version.segments
-    STRING = ActiveModel.version.to_s
+    gem_version
   end
 end