Follow hash syntax for logstash-event v1.4.x

This is a working version of roidrage#75. It will break backwards compability
with < 1.4.x for logstash users.

Gemfile

@@ -5,7 +5,7 @@ gemspec
 
 group :test do
   gem 'actionpack'
-  gem 'logstash-event', '1.1.5'
+  gem 'logstash-event', git: 'git@github.com:elasticsearch/logstash.git', tag: '>1.5'
   gem 'rubocop'
   gem 'lines'
 end

lib/lograge/formatters/logstash.rb

@@ -5,7 +5,7 @@ def call(data)
         load_dependencies
         event = LogStash::Event.new(data)
 
-        event.message = "[#{data[:status]}] #{data[:method]} #{data[:path]} (#{data[:controller]}##{data[:action]})"
+        event['message'] = "[#{data[:status]}] #{data[:method]} #{data[:path]} (#{data[:controller]}##{data[:action]})"
         event.to_json
       end