Update gem dependencies.
Prepare for version 2.0.0.
Remove Ruby 1.8.7 compatibility code.
Modify gemspec file.
Update Rubocop styles.
Update Travis file.
Update README.md file.
Update rake tasks.
Update hash syntax to 1.9 style.
Modify rdoc_options on gemspec file.
Lock gem versions.
Remove JRuby 1.6 from Travis configuration file.
Add `rbx` to Travis configuration file.