Fix up PPI::Token::insert_{after,before}.

I stumbled across this while trying to make
Dist::Zilla::Plugin::PkgVersion work better with perlcritic.  More
info here:

   https://github.com/rjbs/Dist-Zilla/pull/168

The comments for PPI::Token::insert_{after,before} say that they may
only insert statements, but the code inexplicably requires
PPI::Structures.

I think that requiring a Statement is appropriate the right thing,
this commit makes the code match the comment.

The tests in t still pass.

I'll submit a pull request and hope for feedback from folks wise.