Check values passed in Position's constructor

We check that the values are integer values, to properly access board coordinates. Without this, it was possible to pass null or string values, that may have caused unexpected behaviours.
Test Position class instantiation with null values
Delete duplicate Position class
Remove PHP 5.3 testing and support
Move board initialization into board builder
Create Direction
Remove Square class

It was actually unnecessarily complicating things.
Update tests for PositionChecker
Check on PHP 5.6 on travis
Add PHPUnit versions tested up to 4.4 on travis