Fix iconv deprecation message

````
/Users/pmorley/.rvm/gems/ruby-1.9.3-p194@puppet-modules/gems/puppet-parse-0.0.4/lib/vendor/puppet/external/pson/pure.rb:7:in `<module:PSON>': iconv will be deprecated in the future, use String#encode instead.
````

Looks like an issue with puppet 2.7.* and they fixed it in 3.1.*:
https://github.com/puppetlabs/puppet/commit/eea1ef5a594c2c9a33cfafac899b1c4317dabc2a

I assume you don't want to fully upgrade the whole vendor/puppet folder, so I just pulled in the one change needed. What do you think?

BTW, awesome gem, exactly what I needed for an easy CI task for our puppet modules :) :+1: