Use my msgpack fork temporarily for PHP 7.1 compatibility

Author: sodabrew

.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 struct_unserialize_data
         phpize
         ./configure
         make
@@ -237,7 +238,7 @@ case $ACTION in
         # Install igbinary extension
         install_igbinary
 
-        # install msgpack
+        # Install msgpack extension
         install_msgpack
 
         install_memcached