Update composer requirements for php-zip. With PCLZip support added this isn't a requirement any more.

composer.json

@@ -33,8 +33,7 @@
     ],
     "require": {
         "php": ">=5.3.3",
-        "ext-xml": "*",
-        "ext-zip": "*"
+        "ext-xml": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "3.7.*",
@@ -45,6 +44,7 @@
         "mpdf/mpdf": "5.*"
     },
     "suggest": {
+        "ext-zip": "Used to create docx files",
         "ext-gd2": "Required to add images",
         "ext-xmlwriter": "Required to write DOCX and ODT",
         "ext-xsl": "Required to apply XSL style sheet to template part",