* added flake8 and flake8-import-order to travis for py27
* fixed a test that fails from time to time depending on an order of a dict
* flake8 tweaks for the entire codebase excluding tests
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.
