Fix style issue

Closes #30

lib/mutant/killer/forked.rb

@@ -33,9 +33,9 @@ module Mutant
         fork do
           begin
             killer = @killer.new(strategy, mutation)
-            Kernel.exit(killer.success? ? 0 : 1)
+            exit(killer.success? ? CLI::EXIT_SUCCESS : CLI::EXIT_FAILURE)
           rescue
-            Kernel.exit(1)
+            exit(CLI::EXIT_FAILURE)
           end
         end
 

lib/mutant/loader.rb

@@ -40,7 +40,7 @@ module Mutant
       # @api private
       #
       def run
-        Kernel.eval(source, TOPLEVEL_BINDING, @subject.source_path, @subject.source_line)
+        eval(source, TOPLEVEL_BINDING, @subject.source_path, @subject.source_line)
       end
 
       # Return source