Fix ErlangActor loading

Author: pitr-ch

lib-edge/concurrent-edge.rb

@@ -13,4 +13,4 @@
 require 'concurrent/edge/channel'
 
 require 'concurrent/edge/processing_actor'
-require 'concurrent/edge/erlang_actor'
+require 'concurrent/edge/erlang_actor' if Concurrent.ruby_version :>=, 2, 1, 0