Apparently libzstd is no longer installed in travis

Author: Willem Stuursma

.travis.yml

@@ -34,6 +34,8 @@ matrix:
     - php: nightly
 
 before_install:
+  - sudo apt-get update
+  - sudo apt-get -y install libzstd-dev=1.3.1
   - pecl update-channels
   - yes | pecl install -f igbinary lzf redis
   - phpenv config-add tests/php-travis.ini