Update databases/py-barman to 1.6.0.

Version 1.6.0 - 29 Feb 2016

-   Support for streaming replication connection through the
    streaming_conninfo server option
-   Support for the streaming_archiver option that allows Barman to
    receive WAL files through PostgreSQL's native streaming protocol.
    When set to 'on', it relies on pg_receivexlog to receive WAL data,
    reducing Recovery Point Objective. Currently, WAL streaming is an
    additional feature (standard log archiving is still required)
-   Implement the receive-wal command that, when streaming_archiver is
    on, wraps pg_receivexlog for WAL streaming. Add --stop option to
    stop receiving WAL files via streaming protocol. Add --reset option
    to reset the streaming status and restart from the current xlog
    in Postgres.
-   Automatic management (startup and stop) of receive-wal command via
    cron command
-   Support for the path_prefix configuration option
-   Introduction of the archiver option (currently fixed to on) which
    enables continuous WAL archiving for a specific server, through log
    shipping via PostgreSQL's archive_command
-   Support for streaming_wals_directory and errors_directory options
-   Management of WAL duplicates in archive-wal command and integration
    with check command
-   Verify if pg_receivexlog is running in check command when
    streaming_archiver is enabled
-   Verify if failed backups are present in check command
-   Accept compressed WAL files in incoming directory
-   Add support for the pigz compressor (thanks to Stefano Zacchiroli
    zack@upsilon.cc)
-   Implement pygzip and pybzip2 compressors (based on an initial idea
    of Christoph Moench-Tegeder christoph@2ndquadrant.de)
-   Creation of an implicit restore point at the end of a backup
-   Current size of the PostgreSQL data files in barman status
-   Permit archive_mode=always for PostgreSQL 9.5 servers (thanks to
    Christoph Moench-Tegeder christoph@2ndquadrant.de)
-   Complete refactoring of the code responsible for connecting to
    PostgreSQL
-   Improve messaging of cron command regarding sub-processes
-   Native support for Python >= 3.3
-   Changes of behaviour:
    -   Stop trashing WAL files during archive-wal (commit:e3a1d16)
-   Bug fixes:
    -   Atomic WAL file archiving (#9 and #12)
    -   Propagate "-c" option to any Barman subprocess (#19)
    -   Fix management of backup ID during backup deletion (#22)
    -   Improve archive-wal robustness and log messages (#24)
    -   Improve error handling in case of missing parameters

Version 1.5.1 - 16 Nov 2015

-   Add support for the 'archive-wal' command which performs WAL
    maintenance operations on a given server
-   Add support for "per-server" concurrency of the 'cron' command
-   Improved management of xlog.db errors
-   Add support for mixed compression types in WAL files (SF.net#61)
-   Bug fixes:
    -   Avoid retention policy checks during the recovery
    -   Avoid 'wal_level' check on PostgreSQL version < 9.0 (#3)
    -   Fix backup size calculation (#5)


(fhajny)