Fix for 'invalid byte sequence in US-ASCII'

I was getting this error running the readme_spec in Ruby 1.9.3 (p362, OSX Lion).

I believe the fix is to change the default encoding. (Based on what I read [here](https://github.com/railsmachine/moonshine/wiki/ArgumentError%3A-invalid-byte-sequence-in-US-ASCII)). Does this look like a reasonable fix?

Details:
```ruby
rspec spec/readme_spec.rb
Run options:
  include {:focus=>true}
  exclude {:ruby=>#<Proc:./spec/spec_helper.rb:10>}

All examples were filtered out; ignoring {:focus=>true}
......F......F..

  1) README the 'Reading RDF data in the N-Triples format' example should output inspected statements
     Failure/Error: line.should match(/^\#<RDF::Statement:0x[\da-fA-F]+\(.*?\)>\Z/)
     ArgumentError:
       invalid byte sequence in US-ASCII
     # ./spec/readme_spec.rb:74:in `block (4 levels) in <top (required)>'
     # ./spec/readme_spec.rb:73:in `each_line'
     # ./spec/readme_spec.rb:73:in `block (3 levels) in <top (required)>'

  2) README the 'Reading RDF data in the N-Quads format' example should output inspected statements
     Failure/Error: line.should match(/^\#<RDF::Statement:0x[\da-fA-F]+\(.*?\)>\Z/)
     ArgumentError:
       invalid byte sequence in US-ASCII
     # ./spec/readme_spec.rb:136:in `block (4 levels) in <top (required)>'
     # ./spec/readme_spec.rb:135:in `each_line'
     # ./spec/readme_spec.rb:135:in `block (3 levels) in <top (required)>'
```