Use $::postgresql::server::_version

$::postgresql::server::version (w/o underscore) is not set anymore when the
$version parameter is not used in the latest version of the module, so we
have to use _version to get the right value but still be backwards compatible
with postgresql::server's $version parameter.
Merge pull request #30 from ckaenzig/changed-postgresql-version-variable

Use $::postgresql::server::_version