switch to using processor count instead of physical

see grosser/parallel#300
it seems to be faster and it is the same as parallel_tests uses

lib/parallel_split_test.rb

@@ -14,11 +14,7 @@ def choose_number_of_processes
     end
 
     def best_number_of_processes
-      [
-        ENV['PARALLEL_SPLIT_TEST_PROCESSES'],
-        Parallel.physical_processor_count,
-        Parallel.processor_count
-      ].map(&:to_i).find{|number| number > 0 }
+      Integer(ENV['PARALLEL_SPLIT_TEST_PROCESSES'] || Parallel.processor_count)
     end
   end
 end

spec/parallel_split_test_spec.rb

@@ -17,13 +17,7 @@
       expect(count).to eq(5)
     end
 
-    it "uses physical_processor_count" do
-      allow(Parallel).to receive(:physical_processor_count).and_return 6
-      expect(count).to eq(6)
-    end
-
-    it "uses processor_count if everything else fails" do
-      allow(Parallel).to receive(:physical_processor_count).and_return 0
+    it "uses processor_count" do
       allow(Parallel).to receive(:processor_count).and_return 7
       expect(count).to eq(7)
     end