slic3r: update 1.2.1 -> 1.2.6

Release notes: http://slic3r.org/releases/1.2.6

Slic3r needs additional dependency, 'threads'. Add it.

In addition to bug fixes and some new features, this update stops Slic3r
from printing this on startup:

  Running Slic3r under Perl >= 5.16 is not supported nor recommended

We don't have perl < 5.16 anymore, so we better update slic3r.