fix permission fixing code to only apply to published lib code

Rakefile

@@ -7,7 +7,7 @@ Bundler::GemHelper.install_tasks
 # See https://github.com/simplecov-ruby/simplecov/issues/171
 desc "Set permissions on all files so they are compatible with both user-local and system-wide installs"
 task :fix_permissions do
-  system 'bash -c "find . -type f -exec chmod 644 {} \; && find . -type d -exec chmod 755 {} \;"'
+  system 'bash -c "find lib/ -type f -exec chmod 644 {} \; && find . -type d -exec chmod 755 {} \;"'
 end
 # Enforce proper permissions on each build
 Rake::Task[:build].prerequisites.unshift :fix_permissions