Bump version to 19.0, switched to using CalVer

(CalVer is calendar versioning)

src/main.cpp

@@ -58,7 +58,7 @@
 //  \brief Athena++ main program
 
 int main(int argc, char *argv[]) {
-  std::string athena_version = "version 1.1.1 - July 2018";
+  std::string athena_version = "version 19.0 - August 2019";
   char *input_filename = nullptr, *restart_filename = nullptr;
   char *prundir = nullptr;
   int res_flag = 0;   // set to 1 if -r        argument is on cmdline