Fix rubocop style complaints

lib/carto/storage_options/local.rb

@@ -17,7 +17,7 @@ class Carto::StorageOptions::Local
     target_file_path = File.join(target_directory, filename)
     # NOTE: default permissions are 0600, which don't allow nginx to
     # serve them
-    FileUtils.chmod(0644, target_file_path)
+    FileUtils.chmod(0o644, target_file_path)
     url = File.join('/uploads', @location, path, filename)
 
     [target_file_path, url]

spec/lib/carto/storage_options/local_spec.rb

@@ -38,7 +38,7 @@ describe Carto::StorageOptions::Local do
     end
 
     it 'target file should have 0644 perms for nginx to serve them' do
-      (File.stat(@path).mode & 0777).should eq 0644
+      (File.stat(@path).mode & 0o777).should eq 0o644
     end
   end