Update default package version for libpq-dev

The release of PostgreSQL 9.4 pushed 9.4 related packages into the
`main` repository channel and pushed 9.3 related packages to the `9.3`
repository channel. This changeset adds a new attribute for PostgreSQL
repository channel, which is used to set the APT repository channel.
0.2.0
Merge branch 'release/0.2.0' into develop