message: Replace C++11 to_string feature with stringstream
diff: #include "mfem.hpp"
  #include <fstream>
  -#include <string>
  +#include <sstream>
  #include <iostream>
  using namespace std;
  @@ -317,7 +317,9 @@ int main(int argc, char *argv[])
  {
  // Write out the grid function, since GLvis cannot yet
  // visualize vector grid functions over the socket.
  - ofstream osol(string("vortex-") + to_string(ti) + string(".gf"));
  + ostringstream temp;
  + temp << "vortex-" << ti << ".gf";
  + ofstream osol(temp.str());
  osol.precision(precision);
  u.Save(osol);
  }
  