Modernize build environment

- Add Gemfile and gemspec
- Use rake's and bundler's tasks instead of Levitate
- Add .gitignore
Ignore hand-built gem files
Pull in lib_ast_ruby_parser
Remove separate lib_ast_ruby_parser/version.rb file
Move LiveASTRubyParser to LiveAST::RubyParser
Update dependency on ruby_parser

- Change expected sexps to match new sexp format used in ruby_parser 3
- Update README.rdoc with new sexp format
- Fix encoding tests: ruby_parser 3 encodes all strings as utf-8
Add rdoc tasks
Replace boc with binding_of_caller
Prepare mvz-live_ast version 1.1.0 for release