Added link to requirements and fixed upgrade link
Because of #364 (closed) , I worked on the following:
- On the downloads-ee page the requirements link is missing, please link to http://doc.gitlab.com/ee/install/requirements.html
- On the downloads-ee page the update link should be a deeplink to the second part of https://about.gitlab.com/update/ (you might have to add an id on the page)
cc/ @sytses