remove dependency on Marpa for now

The Marpa filter does not yet work, so we don't need to depend on it.

Fixes #48
fix differences between JSON::PP and JSON::XS

JSON::XS does not allow setting the indent length, and doesn't put a
newline after the output.

But now that I think about it, JSON::PP probably shouldn't either...

Fixes #44
fix "yq::filter used only once" warning

This is a warning that I don't get on OS X for some reason, but I do get
on RHEL...

Fixes #51