fix specs by allowing the use of external logstash core

Author: jsvd

Gemfile

@@ -1,4 +1,11 @@
 source 'https://rubygems.org'
 
-# Specify your gem's dependencies in logstash-mass_effect.gemspec
 gemspec
+
+logstash_path = ENV["LOGSTASH_PATH"] || "../../logstash"
+use_logstash_source = ENV["LOGSTASH_SOURCE"] && ENV["LOGSTASH_SOURCE"].to_s == "1"
+
+if Dir.exist?(logstash_path) && use_logstash_source
+  gem 'logstash-core', :path => "#{logstash_path}/logstash-core"
+  gem 'logstash-core-plugin-api', :path => "#{logstash_path}/logstash-core-plugin-api"
+end