Change snapshot testing gem

[apainintheneck]

Currently this project uses the rspec-snapshot gem which works fine but has some major flaws. It doesn't have a way to disable creating new snapshots on CI and tests will pass in that case which is not ideal. There is an older gem called snapshot_testing which would work better in this case.

• Tests pass in CI if no snapshot is found levinmr/rspec-snapshot#32

[apainintheneck]

They seem to be incompatible so it's not worth the effort.