Release 1.6.0

Changelog

0973182 [docker] use go modules
92ba2b1 [travis] enable module-based builds
6ce0043 Update copyright year to 2019
724c751 Migrate to Go modules
264c12d Support output in CSV format
07cb13e Accept null for pg_available_extensions.comment, fixes #21
d4f46ce Handle case when pg_get_expr returns null, fixes #20

Built with Go version 1.11.5.