Update to 0.56

Upstream changes:
0.56  2016-03-16T17:07:05Z
      - Added Firebird support, thanks to Stefan Suciu.
      - Fixed SQLite savepoint support to properly detect SQlite versions 3.9
        and higher.
      - Restored MySQL savepoint testing when the DBICTEST_* environment
        variables are set.

0.55  2016-02-05T18:59:24Z
      - Added versions to the RollbackError exception classes to make PAUSE
        happy.

0.54  2016-02-05T18:52:44Z
      - Removeed the Pod tests from the distribution.
      - Fixed the example code for handling `rollback_error` exceptions in the
        documentation to properly wrap the transaction in an exception
        handler. Patch from Perlover (issue #32).
      - Improved handling of connection failures when RaiseError (or
        HandleError) is not set. Thanks to Andreas Huber for the report and
        fix.
      - Document that `$_` is set locally, not globally, in the methods that
        set it. Suggested by William Lindley.
      - The disconnect method no longer longer remove entries from the DBI
        handle's CachedKids attribute. That behavior appears to be a
        workaround for a database we don't (yet) support.
      - Fixed a test failure where the test system has the `$DBI_DSN` or
        `$DBI_DRIVER` environment variable set. Thanks to Erik Rijkers for the
        patch.
      - Added recommendation to use DBD::Pg 3.5.0 or later to the Pg driver.
        Earlier versions had an incorrect implementation of the `ping()`
        method (Issue #41).


(wen)