asterisk-13.x: bump to 13.18.5

asterisk-13.x: bump to 13.18.5

The currently used version 13.8.0 has the following open CVEs:

CVE-2016-7551
CVE-2016-9938
CVE-2017-14099
CVE-2017-14100
CVE-2017-14603
CVE-2017-16671
CVE-2017-16672
CVE-2017-17090
CVE-2017-17664
CVE-2017-17850
CVE-2017-7617
CVE-2017-9358
CVE-2017-17850

Upstream provided patches for these but used 13.13.0 or later versions
as baseline. Multiple of these patches fail to apply to 13.8.0. So
upgrade to the current version instead.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>