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`.
