Version 2.0.0.beta2.
Version 2.0.0.beta2.

* respond_to_missing in RDF::Query::Solution
* Freeze Literal, Node and URI values on instantiation.
* Freeze `Term#to_base` result.
* Use symbol, rather than string, to index intern cache.
* Change "rdf" executable to take a pipeline of commands, so that multiple commands can be stacked.
* Remove `vocab-fetch`
* Deprecate `Vocabulary.load` in favor of `Vocabulary.from_graph`.
* Change vocabulary generation in Rakefile to use rdf cli.

ruby-rdf/rdf,PerfectMemory/rdf