Using already existing LOGSTASH_DEBUG env variable to debug + update README

Setting DEBUG env variable doesn't change anything for me
Furthermode LOGSTASH_DEBUG variable is already defined in Logstash::Logger, better to reuse it and let interested people know

spec/README.md

@@ -8,3 +8,7 @@ Run them all:
 
   `rspec spec/**/*.rb`
 
+Debug one test:
+
+  `LOGSTASH_DEBUG=y rspec spec/the/test.rb`
+

spec/test_utils.rb

@@ -13,11 +13,7 @@ end
 
 module LogStash
   module RSpec
-    if ENV["DEBUG"] 
-      require "cabin"
-      Cabin::Channel.get.level = :debug
-    end
-
+    
     def config(configstr)
       @config_str = configstr
     end # def config