Revert "Use temporary fork of igbinary for php-8 support"

Upstream accepted the patch for PHP master / PHP 8 support.
This reverts commit 26e38cc.

Author: sodabrew

.travis/travis.sh

@@ -67,9 +67,8 @@ function install_libmemcached() {
 }
 
 function install_igbinary() {
-    git clone https://github.com/sodabrew/igbinary.git
+    git clone https://github.com/igbinary/igbinary.git
     pushd igbinary
-        git checkout php-8
         phpize
         ./configure
         make