use travis php image

.travis.yml

 dist: trusty
 sudo: required
 
-language: cpp
+language: php
 
 addons:
  apt:
  sources:
- - sourceline: 'ppa:ondrej/php'
  - sourceline: 'ppa:ubuntu-toolchain-r/test'
  packages:
- - php7.1-dev
  - autoconf
  - build-essential
  - gcc-5
  - g++-5
 
+php:
+ - '7.1'
+
 before_install:
  - export CC=gcc-5
  - export CXX=g++-5
@@ -23,7 +24,7 @@ before_install:
 script:
  - dpkg -l | grep php
  - ls /usr/bin | grep php
- - phpize7.1
+ - phpize
  - ./configure --enable-bencode
  - make -j$(nproc)
  - make test