Remove µs string before multiply, fix return type

[mihaliak]

Hi,

we are getting errors in calculateElapsedTime method since it is not removing string before multiply.

ErrorException 

  A non-numeric value encountered

  at vendor/laravel/octane/src/Commands/StartRoadRunnerCommand.php:209
    205▕         if (Str::endsWith($elapsed, 'µs')) {
    206▕             return $elapsed * 0.001;
    207▕         }
    208▕ 
  ➜ 209▕         return $elapsed * 1000;
    210▕     }
    211▕ 
    212▕     /**
    213▕      * Stop the server.

This PR also fixes return type and force input string value to be float.