Update the github URL for db.php

[srtfisher]

Update the URL used for db.php to reflect the proper URL from Github: https://raw.githubusercontent.com/markoheijnen/wp-mysqli/master/db.php

At the time of writing, https://raw.github.com/markoheijnen/wp-mysqli/master/db.php is throwing a 503 error (Error 503 hostname doesn't match against certificate). The correct raw URL doesn't 503.

[danielbachhuber]

Thanks @srtfisher !