Simplify and reformat gemspec
Add yard task and rake clean tasks to Rakefile
Fix code-style
Travis: enable containerized (faster) builds
Travis: run on MRI 2.2.0 and 2.1.0
Change required ruby version to >=2.1

Required keyword arguments are not supported in <2.1.
Release v1.5.1
Prepare for next development iteration
Avoid setting backend_name to empty string

When backend_name is empty string, then some code highlighters doesn't
work. Yeah, it's weird...
Update htmlbeautifier to 0.0.11 (fixes #3)
Fix deprecated pattern style 'Rakefile' in .rubocop.yml
Release v1.5.1.1
Prepare for next development iteration
Add basic support for LaTeX backends (WIP)
Change syntax of example's header for LaTeX to be more specific