Bumping the required version of pymongo. We definitely do not work with ...

...1.9 anymore, and we only test against 2.1.1. dgregor ran into an issue where he managed to do a fresh Pulp install with pymongo 1.9 and hit a migration error, so we definitely need to be more accurate with this requirement.

Since 2.1.1 is in EPEL and is what we test against, that seems like a reasonable version to require.