YAAAY! PHP 5.2 support is finally fixed! YAAAY!

Finally, it all works.

The problems were:

- There were some `__DIR__`s, this is not available in PHP 5.2, so it all failed and threw an Exception for a non-existing content dir
- The definitions for content file mime types in default.php were missing, so it could not get that information and returned `null`.

This PR also fixes some other bugs and adds some sugar to mime type detection (Fileinfo and a return value of `false` instead of `null`).

Additionally, it adds PHP 5.5 as allowed failure, so Travis will not call the build failed just because PHP 5.5 failed. PHP 5.5 is Alpha anyway, so not officially supported by Kirby yet.

Last but not least, it adds a try catch around the first `site()` call in `testing_bootstrap.php` to catch exactly the errors I needed to know when finding out what was wrong here. :)