(fact-81) Use Emit.dump rather than to_plist

See https://github.com/puppetlabs/facter/pull/499 and the linked
redmine tickets for backstory. The basic issue here is that
CFPropertyList monkey-patches some core ruby classes with a
to_plist method, which defaults to returning binary data. This
code was originally written to the vendored CFPropertyList
previously included in facter (but unvendored in fact-94),
which provided a different to_plist implementation returning
text data.

This commit avoids the conflicting to_plist implementations by
calling its own dump method, thus no change in behavior.
Merge pull request #625 from kylog/issue/fact-81/avoid-multiple-impls-of-to_plist

(fact-81) Use Emit.dump rather than to_plist