fix ci img in readme [ci skip]

Author: krakjoe

README.markdown

@@ -1,6 +1,6 @@
 Build Status
 ------------
-[![Build Status](https://travis-ci.org/php-memcached-dev/php-memcached.png?branch=master)](https://travis-ci.org/php-memcached-dev/php-memcached)
+[![Build Status](https://travis-ci.org/php-memcached-dev/php-memcached.png?branch=php7)](https://travis-ci.org/php-memcached-dev/php-memcached)
 
 Description
 -----------