force moped v1.2.1

avoids Moped::Errors::OperationFailure "need to login" in edx loadtest environment

full changelog between v1.2.0 and v1.2.1 here: https://github.com/mongoid/moped/compare/v1.2.0...v1.2.1

all rspec tests pass, and 500 errors in loadtest go away.

@e0d @gwprice @kevinchugh