Bump/psycopg2 2.7.5

[drewbanin]

Fixes #773, #704

We originally intended to use the psycopg2-binary dep, but it sounds like that's not ready for production workloads yet. Via the installation docs:

Note The -binary package is meant for beginners to start playing with Python and PostgreSQL without the need to meet the build requirements. If you are the maintainer of a publish package depending on psycopg2 you shouldn’t use psycopg2-binary as a module dependency. For production use you are advised to use the source distribution.

This PR bumps the dep to 2.7.5, which should alleviate issues with installation on Fedora/Arch/etc. It also contains fixes around the closed attribute on connections, which we will likely need to use to manage long-running queries that result in disconnected cursors.