php-vcr issue #67: unit test fail on Windows 7.

Great find! 
Instead of changing the `Configuration` code, I'd rather change the `[isBlacklisted](https://github.com/php-vcr/php-vcr/blob/request_blocking_support/src/VCR/Util/StreamProcessor.php#L124)` method. Before calling `strpos` the paths could be normalised using your already implemented logic in `convertToOsSlash`. This has some advantages:
 * keeps the `Configuration` a dumb object, just checking and storing values
 * separations of concern: the check if a path it blacklisted is in a single method (`isBlacklisted`)
 * I'm pretty sure the `isWhitelisted` has the same problem