Avoid need to force-encode strings

By setting the encoding for the entire file to utf-8, the empty string literals
used in SexpHandlers::Literals#extract_string_parts already have the correct
encoding.
Run ParseTree tests as part of the default suite
Fix test description
Fix test for CommentingRipperParser to expect sexps as output

While the equality tests will not fail for equivalent arrays and sexps, the
output when the results do not match is not well suited for comparison by
eye.
Make tests pass on Ruby 2.0.0
Fix version check to work for Ruby 2.1
Add Ruby 2.0.0 and 2.1.0 to the Travis CI configuration
Convert README to markdown format
Adjust gemspec to include README.md
Update README

- Update statement of supported versions of Ruby
- Update Copyright statement