The Mongo storage driver in Anemone hasn't been updated since 2010 and still uses the syntax for the 1.x version of the Mongo ruby driver.
The easiest way to work around the problem is to instruct your script to use the older gem.
First, check if you have a 1.x version of the gem on your system. From the command line:
gem list mongo
It should give you something like:
*** LOCAL GEMS ***
mongo (2.2.5, 1.12.5)
mongodb (2.1.0)
I had a 1.x version (1.12.5), so I'm good to go. If you don't, you can install one using:
gem install mongo -v 1.12.5
Then, instruct your script to use the 1.x version of the gem by placing this line at the top of your file:
gem 'mongo', '=1.12.5'
Note, if you're using Mongo anywhere else in your script, it will now need to use the 1.x syntax.
Also, you may get a warning asking you to install bson_ext, which you can do with:
gem install bson_ext