message: fix warning in compiling core
  remove reference to deprecated StyledStreamWriter
diff: @@ -71,9 +71,8 @@ TEST_F(PolicyManagerImplTest2, UpdatedPreloadedPT_ExpectLPT_IsUpdated) {
  }
  ifile.close();
  - Json::StyledStreamWriter writer;
  std::ofstream ofile(preloaded_pt_filename_);
  - writer.write(ofile, root);
  + ofile << root;
  ofile.flush();
  ofile.close();
  