display if inspec version is outdated

Signed-off-by: Christoph Hartmann <chris@lollyrock.com>

lib/inspec/cli.rb

@@ -9,6 +9,7 @@ require 'thor'
 require 'json'
 require 'pp'
 require 'utils/json_log'
+require 'utils/latest_version'
 require 'inspec/base_cli'
 require 'inspec/plugins'
 require 'inspec/runner_mock'
@@ -223,6 +224,11 @@ class Inspec::InspecCLI < Inspec::BaseCLI # rubocop:disable Metrics/ClassLength
   desc 'version', 'prints the version of this tool'
   def version
     puts Inspec::VERSION
+    # display outdated version
+    latest = LatestInSpecVersion.new.latest
+    if Gem::Version.new(Inspec::VERSION) < Gem::Version.new(latest)
+      puts "\nYour version of InSpec is out of date! The latest version is #{latest}."
+    end
   end
 
   private

lib/utils/latest_version.rb

@@ -0,0 +1,20 @@
+# encoding: utf-8
+# author: Christoph Hartmann
+
+require 'json'
+require 'net/http'
+
+class LatestInSpecVersion
+  # fetches the latest version from rubygems server
+  def latest
+    uri = URI('https://rubygems.org/api/v1/gems/inspec.json')
+    res = Net::HTTP.start(uri.host, uri.port, use_ssl: uri.scheme == 'https') {|http|
+      http.read_timeout = 0.5
+      http.get(uri.path)
+    }
+    inspec_info = JSON.parse(res.body)
+    inspec_info['version']
+  rescue Exception # rubocop:disable Lint/RescueException
+    nil
+  end
+end