add post install message warning about dropping 1.8.7 support

Fixes #154

Author: oreoshake

lib/secure_headers/version.rb

@@ -1,3 +1,3 @@
 module SecureHeaders
-  VERSION = "2.2.3"
+  VERSION = "2.2.4"
 end

secure_headers.gemspec

@@ -20,4 +20,5 @@ Gem::Specification.new do |gem|
   gem.require_paths = ["lib"]
   gem.add_development_dependency "rake"
   gem.add_dependency "user_agent_parser"
+  gem.post_install_message = "Heads up, secure_headers Ruby 1.8.7 support will go away in the next major release."
 end