Allow running from older Rubies with warning

Author: marcandre

lib/mspec/utils/script.rb

@@ -39,7 +39,7 @@ def self.get(key)
 
  def initialize
  ruby_version_is ""..."2.5" do
- abort "MSpec needs Ruby 2.5 or more recent"
+ warn "MSpec is supported for Ruby 2.5 and above only"
  end
 
  config[:formatter] = nil