Add memcached extension support for PHP 7.3 (#2195)

php-fpm/Dockerfile

@@ -353,7 +353,7 @@ RUN if [ ${INSTALL_MEMCACHED} = true ]; then \
     if [ $(php -r "echo PHP_MAJOR_VERSION;") = "5" ]; then \
       curl -L -o /tmp/memcached.tar.gz "https://github.com/php-memcached-dev/php-memcached/archive/2.2.0.tar.gz"; \
     else \
-      curl -L -o /tmp/memcached.tar.gz "https://github.com/php-memcached-dev/php-memcached/archive/php7.tar.gz"; \
+      curl -L -o /tmp/memcached.tar.gz "https://github.com/php-memcached-dev/php-memcached/archive/master.tar.gz"; \
     fi \
     && mkdir -p memcached \
     && tar -C memcached -zxvf /tmp/memcached.tar.gz --strip 1 \