use Mojo::JSON for tweet extraction now

I love HTML::TreeBuilder::XPath, but after testing some benchmarks
that I wrote, I have decided that Mojo::JSON is MUCH faster, and
there is not much of a noticable delay when the bot replies with
a Vim tip.  So I will be including this in v0.03 I think
require perl 5.6
remove HTML::TreeBuilder::XPath, require Mojo::JSON