only run the EM specs if EM is installed

spec/em/em_spec.rb

@@ -1,4 +1,5 @@
 # encoding: UTF-8
+if defined? EventMachine
   require 'spec_helper'
   require 'mysql2/em'
 
@@ -43,3 +44,6 @@ describe Mysql2::EM::Client do
       results[1].keys.should include("second_query")
     end
   end
+else
+  puts "EventMachine not installed, skipping the specs that use it"
+end