Disable GC before doing performance test

spec/lib/object_serializer_performance_spec.rb

@@ -4,6 +4,9 @@ describe FastJsonapi::ObjectSerializer, performance: true do
   include_context 'movie class'
   include_context 'ams movie class'
 
+  before(:all) { GC.disable }
+  after(:all) { GC.enable }
+
   context 'when testing performance of serialization' do
     it 'should create a hash of 1000 records in less than 50 ms' do
       movies = 1000.times.map { |_i| movie }