Make requirement for PyMongo>=2.5 more consistent

Commit 7aa1f473785ed17cff280835285a69e401fd9b86 requires PyMongo >= v2.5.
This updates the requirements file to make this requirement explicit to
package managers.

Commit 29309dac9a926077962fc0778e5b2fbaf1d29cc2 removed some legacy
compatibility code that would run only with versions of PyMongo < 2.1. The
options 'is_slave' and 'slaves' for register_connection were only used in
this compatibility code, so they are removed too.