Drop --without-libzip option from Travis

We need to build against the system libzip now.

Also add an UPGRADING note.

Author: nikic

UPGRADING

@@ -65,6 +65,10 @@ PHP 7.4 UPGRADE NOTES
     results of Reflection...::getModifiers(), using hard-coded numeric values.
     Use corresponding constants instead (e.g. ReflectionMethod::IS_PUBLIC).
 
+- Zip:
+  . The bundled libzip library has been removed. A system libzip >= 0.11 is now
+    necessary to build the extension.
+
 ========================================
 10. New Global Constants
 ========================================

travis/compile.sh

@@ -45,7 +45,6 @@ $TS \
 --with-png-dir=/usr \
 --enable-exif \
 --enable-zip \
---without-libzip \
 --with-zlib \
 --with-zlib-dir=/usr \
 --enable-soap \