Update required pymongo version
Update required pymongo version

Updated required pymongo version to be at least 2.6 to better
support bulk inserts.

Fixes https://github.com/openelections/core/issues/192

openelections/openelections-core,datamade/openelections-core,cathydeng/openelections-core,datamade/openelections-core,cathydeng/openelections-core,openelections/openelections-core