require 'yardstick/version'

Because require hashes based on the string, not the expanded path of the
file, require the version file the same way that it will be required
elsewhere to avoid constant warnings.
Update backports dependency to ~> 3.0.3
Specify bundler development dependency

Move other development dependencies to Gemfile; bootstrap with bundler.
Use pure Ruby Markdown parser to allow development on JRuby