Rearrange Cucumber and add some flair.

* Move step_definitions/jekyll.rb to just step_definitions.rb
* Rename the formatter to Jekyll::Cucumber::Formatter, it's Jekyll's.
* Add some flair; switch to checks!
* Rename env.rb to helpers.rb

script/cucumber

@@ -1,11 +1,4 @@
 #!/usr/bin/env bash
 
-if ruby --version | grep -q "jruby"
-then
-  echo "Move along, we are not testing features on JRuby right now."
-  exit 0
-else
-  time ruby -S bundle exec cucumber \
-    -f Features::Support::Overview \
-    "$@"
-fi
+time ruby -S bundle exec cucumber \
+  -f Jekyll::Cucumber::Formatter "$@"