Fix lines() cannot split big number

[takyam-git]

>>> \Stringy\Stringy::create('111111111111111111111')->lines();
PHP Warning:  mb_split() expects parameter 3 to be integer, float given in /path/to/vendor/danielstjules/stringy/src/Stringy.php on line 1288

>>> echo 1111111111111111111111111111111
1.1111111111111E+30

Maybe split() 2nd argument does not need.