Error message on startup

When installing pgcli with `sudo apt-get install pgcli`, and running pgcli with the following command:
`pgcli -h 127.0.0.1 -U postgres --password some_database`
then the following error message appears:

```
Exception in thread completion_refresh:
Traceback (most recent call last):
  File "/usr/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/share/pgcli/pgcli/completion_refresher.py", line 71, in _bg_refresh
    refresher(completer, executor)
  File "/usr/share/pgcli/pgcli/completion_refresher.py", line 121, in refresh_functions
    completer.extend_functions(executor.functions())
  File "/usr/share/pgcli/pgcli/pgcompleter.py", line 190, in extend_functions
    for f in func_data:
  File "/usr/share/pgcli/pgcli/pgexecute.py", line 564, in functions
    cur.execute(query)
psycopg2.ProgrammingError: column p.proisagg does not exist
LINE 8: p.proisagg is_aggregate,
                                ^
HINT: Perhaps you meant to reference the column "p.prolang".

```

This does not seem to cause problems - I can use the program. But it should be fixed nonetheless.

Description: Ubuntu 18.04.3 LTS
Release: 18.04

pgcli:
  Installed: 1.6.0-1
  Candidate: 1.6.0-1
  Version table:
 *** 1.6.0-1 500
        500 http://ee.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        500 http://ee.archive.ubuntu.com/ubuntu bionic/universe i386 Packages
        100 /var/lib/dpkg/status

I expected there to be no error on program start, but there was an error.