only run the EM specs if EM is installed

spec/em/em_spec.rb

@@ -1,8 +1,9 @@
 # encoding: UTF-8
-require 'spec_helper'
-require 'mysql2/em'
+if defined? EventMachine
+  require 'spec_helper'
+  require 'mysql2/em'
 
-describe Mysql2::EM::Client do
+  describe Mysql2::EM::Client do
     it "should support async queries" do
       results = []
       EM.run do
@@ -42,4 +43,7 @@ describe Mysql2::EM::Client do
       results[0].keys.should include("first_query")
       results[1].keys.should include("second_query")
     end
+  end
+else
+  puts "EventMachine not installed, skipping the specs that use it"
 end