SimpleCov has a neat new feature since version 0.7.0! You can now tell it to fail builds if coverage has dropped. It's pretty simple to do, just add one line to your SimpleCov config:
SimpleCov.start do # you old config here refuse_coverage_drop end
Now run your tests. You should have a new file "coverage/.last_run.json" which contains the last test coverage. Add this to your repository and commit. Your coworkers will get failed builds whenever they let coverage decrease.
To accept small drops you can use
maximum_coverage_drop instead of
refuse_coverage_drop like this
SimpleCov.start do # you old config here maximum_coverage_drop 1 # = allow 1% coverage drop end