Wrong PIE extension name

[macintoshplus]

In the file composer.json the extension name starts with ext-.

https://github.com/mongodb/mongo-php-driver/blob/v1.x/composer.json#L19

According to the php/pie documentation, the extension name should not be prefixed with ext-.

[alcaeus]

We have released 1.20.1 which brings the correct extension name as well as some other fixes for PIE support.