Relax dependency restrictions and add build matrix

[dzunk]

I've been developing for latest Ruby and Redis (4+), but if we can support Ruby 2.5.x and Redis 3.x, I don't see any reason not to do so.

Ruby 2.4 and below are EOL, so probably not going to worry about them.

Whichever combinations of versions make sense to support, need to add a build matrix to run specs against all of them, instead of just the latest ones.

May also want to pin the redis-time-series docker container version, or include that as part of the matrix as well.