Make the link to a project's forum optional

Omit the link to the forum if the project's metadata does not provide
a forum url
Merge pull request #5 from wilkinsona/optional-forum-link

Make the link to a project's forum optional
Merge branch 'gh-pages' of https://github.com/spring-projects/gh-pages into gh-pages

Conflicts:
	_includes/billboard.html