Deal with redis-rb warning for pipelined call

redis-rb 4.6 deprecated calling `pipelined` without having the given
block accept the client and added a warning.
See https://github.com/redis/redis-rb/blob/21ec1dec064f43def1f47983431861e6491b770e/CHANGELOG.md#460
for more details.

As suggested in their changelog this adapts the call to `pipelined`
accordingly.
This behaviour was also already accepted in the older redis-rb versions,
so it is safe to make this change without limiting the redis-rb
gem versions that can be used together with gcra-ruby.

lib/gcra/redis_store.rb

@@ -30,9 +30,9 @@ def initialize(redis, key_prefix, options = {})
     # Returns the value of the key or nil, if it isn't in the store.
     # Also returns the time from the Redis server, with microsecond precision.
     def get_with_time(key)
-      time_response, value = @redis.pipelined do
-        @redis.time # returns tuple (seconds since epoch, microseconds)
-        @redis.get(@key_prefix + key)
+      time_response, value = @redis.pipelined do |pipeline|
+        pipeline.time # returns tuple (seconds since epoch, microseconds)
+        pipeline.get(@key_prefix + key)
       end
       # Convert tuple to nanoseconds
       time = (time_response[0] * 1_000_000 + time_response[1]) * 1_000