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`.

1 files changed, 15 insertions, 0 deletions

diff --git a/actionmailer/lib/action_mailer/gem_version.rb b/actionmailer/lib/action_mailer/gem_version.rb
new file mode 100644
index 0000000000..b564813ccf
--- /dev/null
+++ b/actionmailer/lib/action_mailer/gem_version.rb
@@ -0,0 +1,15 @@
+module ActionMailer
+  # Returns the version of the currently loaded ActionMailer 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