Use my msgpack branch for PHP 7.3

Author: sodabrew

Diff for: .travis/travis.sh

@@ -77,8 +77,9 @@ function install_igbinary() {
 }
 
 function install_msgpack() {
-    git clone https://github.com/msgpack/msgpack-php.git
+    git clone https://github.com/sodabrew/msgpack-php.git
     pushd msgpack-php
+        git checkout php-7.3
         phpize
         ./configure
         make