add ragel_check rake task to ensure a proper version of ragel is on the path. Fixes #74
according to @whitequark, specific version of ragel is needed; this locks it down to ~>6.8, which is what is needed according to the README.
Merge pull request #74 from yaauie/ragel-dependency-check

`rake test` fails in confusing way when command-line dependency `ragel` not present.