load hiredis to avoid security error

`Security Error Insecure operation - getaddrinfo` happens when
`Redis.new` using `REDIS_URL` within `$SAFE=1`

see also: tdiary/tdiary-cache-redis#2

Author: minimum2scp

build/Gemfile.local

@@ -1,5 +1,7 @@
 gem "puma"
 gem "mysql2"
+gem "hiredis"
+gem "redis", require: ["redis", "redis/connection/hiredis"]
 
 gem "tdiary",                  path: "."
 gem "tdiary-contrib",          github: "tdiary/tdiary-contrib"