rename lowercase grunt file for generate

Fixes error when using 
$ ./node_modules/.bin/actionhero generate

fs.js:427
  return binding.open(pathModule._makeLong(path), stringToFlags(flags), mode);
                 ^
Error: ENOENT, no such file or directory '/home/project/node_modules/actionhero/bin/../grunt/actionhero_gruntfile.js'