use Puppet::Util::Execute.execute with puppet >= 3.4

Resolves this warning on puppet 3.4.2:

    Warning: Puppet::Util::SUIDManager.run_and_capture is deprecated; please
    use Puppet::Util::Execution.execute instead.

This patch also fixes the postgresql_psql provider's broken puppet "4"
support.
Merge pull request #340 from jhoblitt/fix_dep_warning

use Puppet::Util::Execute.execute with puppet >= 3.4