Sereal::Encoder: Don't have skip() fail if we have the wrong decoder version

This fixes a bug in 2b23062, whenever we have skip() it must be in a
SKIP label. Fixes this fail whale I get on my system with an outdated
decoder:

    $ perl -Mblib -Ilib t/120_hdr_data.t
    ok 1 - Encode 12 in header, -16 in body
    ok 2 - OO: Encode 12 in header, -16 in body
    # Could not load correct version of Sereal::Decoder for testing (got: 0.15, needed any of 0.34, 0.24, 0.27, 0.30, 0.31, 0.25, 0.26, 0.33, 0.32, 0.29, 0.23, 0.28, 0.36, 0.35)
    ok 3 # skip Did not find right version of decoder
    Label not found for "last SKIP" at /home/v-perlbrew/perl5/perlbrew/perls/perl-5.14.0/lib/5.14.0/Test/More.pm line 1256.