Expose Jekyll version to templates

Make available `site.version` to liquid
Use proper Ruby syntax for constant
Merge branch 'patch-1' of git://github.com/pathawks/jekyll into pathawks-patch-1

* 'patch-1' of git://github.com/pathawks/jekyll:
  Use proper Ruby syntax for constant
  Expose Jekyll version to templates
Put version under 'jekyll' namespace.
Add cucumber test for `jekyll.version`.
Merge pull request #1481 from mojombo/pathawks-patch-1

Expose Jekyll version to templates: `jekyll.version`