fix `QC::Conn#connect` when url has no host.

This is the case for the default url when running the tests:

        postgres:///queue_classic_test

This fixes the following error when running the tests without configuring the database URL:

```
NoMethodError: undefined method `gsub' for nil:NilClass
    /Users/senny/Projects/queue_classic/lib/queue_classic/conn.rb:89:in `connect'
    /Users/senny/Projects/queue_classic/lib/queue_classic/conn.rb:68:in `connection'
    /Users/senny/Projects/queue_classic/lib/queue_classic/conn.rb:13:in `block in execute'
    <internal:prelude>:10:in `synchronize'
    /Users/senny/Projects/queue_classic/lib/queue_classic/conn.rb:9:in `execute'
    /Users/senny/Projects/queue_classic/test/helper.rb:22:in `init_db'
    /Users/senny/Projects/queue_classic/test/helper.rb:14:in `setup'
```