Check to see if local file exists before opening

The `RDF::Util::File.open_file` method was throwing an exception when the local file did not exist. This modification will check to see if it exists, avoiding the exception and allowing open-uri to download the file.

I'm also getting errors when running the specs. Many of them look like formatting problems, EX:

```
40) JSON::LD test suite toRdf toRdf-0026-in.jsonld: Test creation of multiple types
Failure/Error: quads.sort.join("").should produce(sorted_expected, t.debug)
  Expected: _:b0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Class> .
  _:b0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Resource> .
  
  Actual  : _:t0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Class> .
  _:t0 <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2000/01/rdf-schema#Resource> .
```