mongodb/mongo-ruby-driver — comment on commit 9e2f562892cf339275d2e7f835268c64723daf33
File:  line 0
By fullbridge-batkins:

Downgrade to `1.5.2`. Alternatively you could fork the gem and then edit the `.gemspec` file yourself and modify the line as I have suggested. Fairly certain that would work, although I'd prefer to see the authors fix this. I also sent a support request to Heroku asking if they could upgrade the Rubygems version on their environment.