Add MSPEC_FATAL_DEPRECATION to make MSpec deprecations raise

Author: eregon

lib/mspec/utils/deprecate.rb

@@ -1,6 +1,10 @@
 module MSpec
+  FATAL_DEPRECATION = ENV['MSPEC_FATAL_DEPRECATION']
+
   def self.deprecate(what, replacement)
     user_caller = caller.find { |line| !line.include?('lib/mspec') }
-    $stderr.puts "\n#{what} is deprecated, use #{replacement} instead.\nfrom #{user_caller}"
+    message = "\n#{what} is deprecated, use #{replacement} instead.\nfrom #{user_caller}"
+    $stderr.puts message
+    raise SpecExpectationNotMetError, message if FATAL_DEPRECATION
   end
 end