Update minimum version of psycopg2 required

[karenc]

For the super_user context manager, we are doing something like:

db_conn = psycopg2.connect('user=tester dbname=testing', user='postgres')

This was not supported by psycopg2 < 2.7:

Traceback (most recent call last):
  File \"/var/cnx/venvs/publishing/bin/dbmigrator\", line 11, in <module>
    sys.exit(main())
  File \"/var/cnx/venvs/publishing/local/lib/python2.7/site-packages/dbmigrator/cli.py\", line 104, in main
    return args['cmmd'](**args)
  File \"/var/cnx/venvs/publishing/local/lib/python2.7/site-packages/dbmigrator/utils.py\", line 145, in wrapper
    return func(cursor, *args, **kwargs)
  File \"/var/cnx/venvs/publishing/local/lib/python2.7/site-packages/dbmigrator/commands/migrate.py\", line 32, in cli_command
    run_deferred)
  File \"/var/cnx/venvs/publishing/local/lib/python2.7/site-packages/dbmigrator/utils.py\", line 227, in compare_schema
    callback(*args, **kwargs)
  File \"/var/cnx/venvs/publishing/local/lib/python2.7/site-packages/dbmigrator/utils.py\", line 257, in run_migration
    migration.up(cursor)
  File \"../../var/cnx/venvs/publishing/src/cnx-db/cnxdb/migrations/20170912134157_shred-colxml-uuids.py\", line 19, in up
    with super_user() as super_cursor:
  File \"/usr/lib/python2.7/contextlib.py\", line 17, in __enter__
    return self.gen.next()
  File \"/var/cnx/venvs/publishing/local/lib/python2.7/site-packages/dbmigrator/utils.py\", line 57, in super_user
    user=super_user) as db_conn:
  File \"/var/cnx/venvs/publishing/local/lib/python2.7/site-packages/psycopg2/__init__.py\", line 155, in connect
    % items[0][0])
TypeError: 'user' is an invalid keyword argument when the dsn is specified

[reedstrm]

looks good

[coveralls]

Coverage remained the same at 97.118% when pulling 52458cb on update-psycopg2 into c8a4c96 on master.