Update Rakefile to get correct exit code

The previous method always returned 0 even if tests failed.

Rakefile

@@ -1,11 +1,11 @@
 require 'rake'
+require 'rubygems'
+require 'rspec/core/rake_task'
 
 task :default => [:spec]
 
 desc "Run all module spec tests (Requires rspec-puppet gem)"
-task :spec do
-  system("rspec spec/**/*_spec.rb")
-end
+RSpec::Core::RakeTask.new(:spec)
 
 desc "Build package"
 task :build do