fix travis build, msgpack changed structure

Author: krakjoe

.travis/travis.sh

@@ -79,7 +79,6 @@ function install_igbinary() {
 function install_msgpack() {
     git clone https://github.com/msgpack/msgpack-php.git
     pushd msgpack-php
-        git checkout php7
         phpize
         ./configure
         make