Tighten up psycopg2 version requirement

Author: chadwhitacre

postgres/__init__.py

@@ -9,6 +9,8 @@
 
     $ pip install postgres
 
+:py:mod:`postgres` requires :py:mod:`psycopg2` version 2.5 or higher.
+
 We `test <https://travis-ci.org/gittip/postgres.py>`_ against Python 2.6, 2.7,
 3.2, and 3.3. We don't yet have a testing matrix for different versions of
 :py:mod:`psycopg2` or PostgreSQL.

setup.py

@@ -6,5 +6,5 @@
      , url='https://postgres-py.readthedocs.org'
      , version='2.0.0-dev'
      , packages=find_packages()
-     , install_requires=['psycopg2 >= 2.0.0']
+     , install_requires=['psycopg2 >= 2.5.0']
       )