如何安装 ext-redis [ 未指定版本 ]

composer show --platform

结果：

1. > composer-plugin-api 1.1.0    The Composer Plugin API
1. > ext-bcmath          7.1.16   The bcmath PHP extension
1. > ext-bz2             7.1.16   The bz2 PHP extension
1. > ext-calendar        7.1.16   The calendar PHP extension
1. > ext-ctype           7.1.16   The ctype PHP extension
1. > ext-curl            7.1.16   The curl PHP extension
1. > ext-date            7.1.16   The date PHP extension
1. > ext-dba             7.1.16   The dba PHP extension
1. > ext-dom             20031129 The dom PHP extension
1. > ext-exif            7.1.16   The exif PHP extension
1. > ext-fileinfo        1.0.5    The fileinfo PHP extension
1. > ext-filter          7.1.16   The filter PHP extension
1. > ext-ftp             7.1.16   The ftp PHP extension
1. > ext-gd              7.1.16   The gd PHP extension
1. > ext-hash            1.0      The hash PHP extension
1. > ext-iconv           7.1.16   The iconv PHP extension
1. > ext-json            1.5.0    The json PHP extension
1. > ext-ldap            7.1.16   The ldap PHP extension
1. > ext-libxml          7.1.16   The libxml PHP extension
1. > ext-mbstring        7.1.16   The mbstring PHP extension
1. > ext-mysqli          7.1.16   The mysqli PHP extension
1. > ext-mysqlnd         0        The mysqlnd PHP extension (actual version: mysqlnd 5.0.12-dev - 20150407 - $Id: 38fea24f2847fa7519001be390c98ae0acafe387 $)
1. > ext-openssl         7.1.16   The openssl PHP extension
1. > ext-pcre            7.1.16   The pcre PHP extension
1. > ext-PDO             7.1.16   The PDO PHP extension
1. > ext-pdo_mysql       7.1.16   The pdo_mysql PHP extension
1. > ext-pdo_pgsql       7.1.16   The pdo_pgsql PHP extension
1. > ext-pdo_sqlite      7.1.16   The pdo_sqlite PHP extension
1. > ext-pgsql           7.1.16   The pgsql PHP extension
1. > ext-Phar            2.0.2    The Phar PHP extension
1. > ext-posix           7.1.16   The posix PHP extension
1. > ext-readline        7.1.16   The readline PHP extension
1. > ext-Reflection      7.1.16   The Reflection PHP extension
1. > ext-session         7.1.16   The session PHP extension
1. > ext-shmop           7.1.16   The shmop PHP extension
1. > ext-SimpleXML       7.1.16   The SimpleXML PHP extension
1. > ext-snmp            0.1      The snmp PHP extension
1. > ext-soap            7.1.16   The soap PHP extension
1. > ext-sockets         7.1.16   The sockets PHP extension
1. > ext-SPL             7.1.16   The SPL PHP extension
1. > ext-sqlite3         7.1.16   The sqlite3 PHP extension
1. > ext-sysvmsg         7.1.16   The sysvmsg PHP extension
1. > ext-sysvsem         7.1.16   The sysvsem PHP extension
1. > ext-sysvshm         7.1.16   The sysvshm PHP extension
1. > ext-tidy            7.1.16   The tidy PHP extension
1. > ext-tokenizer       7.1.16   The tokenizer PHP extension
1. > ext-wddx            7.1.16   The wddx PHP extension
1. > ext-xml             7.1.16   The xml PHP extension
1. > ext-xmlreader       7.1.16   The xmlreader PHP extension
1. > ext-xmlrpc          7.1.16   The xmlrpc PHP extension
1. > ext-xmlwriter       7.1.16   The xmlwriter PHP extension
1. > ext-xsl             7.1.16   The xsl PHP extension
1. > ext-zip             1.13.5   The zip PHP extension
1. > ext-zlib            7.1.16   The zlib PHP extension
1. > lib-curl            7.54.0   The curl PHP library
1. > lib-iconv           1.11     The iconv PHP library
1. > lib-libxml          2.9.4    The libxml PHP library
1. > lib-openssl         2.2.7    LibreSSL 2.2.7
1. > lib-pcre            8.40     The pcre PHP library
1. > lib-xsl             1.1.29   The xsl PHP library
1. > php                 7.1.16   The PHP interpreter
1. > php-64bit           7.1.16   The PHP interpreter, 64bit
1. > php-ipv6            7.1.16   The PHP interpreter, with IPv6 support

就是没有 ext-redis，怎么安装上？

composer show --platform 是获取那里的列表？