gemspec: be more specific about dependencies

[kyrofa]

We have run into a number of compatibility issues with this gem, between different Ruby versions and dependency versions. Most recently #1366. I would love to see this gem be more specific about the versions of ruby required, and put an upper bound on its dependencies. In general, it would be ideal to have this gem only specify that it supports the versions of ruby and versions of dependencies that it actually has under test (generally via a test matrix with various Ruby versions and Appraisals). Without this, it puts the onus on consumers of this gem to actually add transient dependency specifiers (e.g. lock connection_pool to < 3 just because react-rails isn't actually compatible with v3 but claims to be, etc.).

I would be happy to do this work, but wanted to get buy-off from maintainers before proposing. It admittedly changes the maintenance game a little.