Truncate _config.php

_config.php without code does not need to contain any text, not even the
<?php tag.
Correct transpositions in text
Update version constraint to allow SilverStripe 3
Clean up if/elseif syntax
Detect dev mode correctly

- Use Director::isDev()
- Eliminate call to deprecated ereg()
Merge pull request #7 from oddnoc/silverstripe3-fixes

Silverstripe 3 fixes