Restrict perlcritic theme to core

to avoid pulling in random (undesired) themes that may be installed.

Without it I get this output on my machine:

    t/author-critic.t ............ 2/2 
    #   Failed test 'Test::Perl::Critic for "blib/lib/Dist/Zilla/Plugin/Test/PodSpelling.pm"'
    #   at /home/rando/perl5/perlbrew/perls/5.14.2-st/lib/site_perl/5.14.2/Test/Perl/Critic.pm line 110.
    # 
    # Perl::Critic found these violations in "blib/lib/Dist/Zilla/Plugin/Test/PodSpelling.pm":
    # [Bangs::ProhibitVagueNames] Variable named "$data" at line 55, near '$self, $data'.  (Severity: 3)
    # [Bangs::ProhibitVagueNames] Variable named "$data" at line 57, near ''attempting stopwords extraction from: ' . $data'.  (Severity: 3)
    # [Bangs::ProhibitVagueNames] Variable named "$data" at line 59, near 'my ( $word ) = $data =~ /(\p{Word}{2,})/xms;'.  (Severity: 3)
    # Looks like you failed 1 test of 2.
    t/author-critic.t ............ Dubious, test returned 1 (wstat 256, 0x100)