Symfony requirement for the realpath_cache_size works with string values

`realpath_cache_size` can be set using so-called *string values* (such as `16K`)
(see http://php.net/manual/en/ini.core.php#ini.sect.performance - default is `16K`)

**Problem**: When such value is detected and compared to 1024 after its conversion to int, it fails although the value is fine for Symfony.

**Solution**: This PR adds a conversion function based on what http://php.net/manual/en/function.ini-get.php recommends.

Original PR: https://github.com/sensiolabs/SensioDistributionBundle/pull/106
Original commit: https://github.com/sensiolabs/SensioDistributionBundle/commit/cf0179711b24d84d4a29d71a4010540f4c990bd8
Related: https://github.com/sensiolabs/SensioDistributionBundle/issues/172, https://github.com/sensiolabs/SensioDistributionBundle/pull/153
That looks like a hack to me. Instead, what about changing the ini requirement to a regular one and do the conversion in a dedicated method like done for some other checks?