Update tox.ini for the new required version of psycopg2

issue5391

tox.ini

@@ -4,7 +4,7 @@ envlist = {py27,py33,py34,py35}-{sqlite,postgresql,mysql},pypy-{sqlite,postgresq
 [testenv]
 commands = {envpython} setup.py test
 deps =
-    {py27,py33,py34,py35}-postgresql: psycopg2 >= 2.0
+    {py27,py33,py34,py35}-postgresql: psycopg2 >= 2.5
     pypy-postgresql: psycopg2cffi >= 2.5
     mysql: MySQL-python
 setenv =