Small fix for rake tasks when using PostgreSQL

[tonyfg]

rake db:drop and db:create are not working when the environment is configured for postgresql with login/password authentication.

The fix allows this scenario to work without affecting configurations that use other auth mechanisms.

[probablykabari]

I'll take a gander, thanks!

[tonyfg]

No problem man :)

There is another way to feed the password to createdb and dropdb but it requires creating a ~/.pgpass file -- we might be overwriting a previous .pgpass file created by the user and that would be bad.
Setting the PGPASSWORD environment variable seems to be the only sane way to use the password from database.yml, especially if there are environments with different database passwords.

If you find any issues with the fix, just let me know and I'll try to find a different way to get around the problem.

[tpitale]

I think this would have passed the build if it weren't for some other thing.

If you'd like me to, @tonyfg, I'll merge this.