Drop support for PclZip

Over the years PclZip became a maintenance problem. It's code base
is old and the official project seems to have died out. ZipArchive
is now more commonly available and offer an easier and more complete
API. So PclZip was dropped in order to focus our efforts on what matters.

Closes #18

phpunit.xml.dist

@@ -14,7 +14,6 @@
         <whitelist>
             <directory suffix=".php">./src</directory>
             <exclude>
-                <directory>./src/PhpSpreadsheet/Shared/PCLZip</directory>
                 <directory>./src/PhpSpreadsheet/Shared/JAMA</directory>
                 <directory>./src/PhpSpreadsheet/Writer/PDF</directory>
             </exclude>