Restore v1.5 index page.

Copied from
https://raw.githubusercontent.com/bundler/bundler-site/1b617fa496e8f5818eb923770bc2dcf2e947cfa0/source/index.haml

Fixes #124.
Merge pull request #128 from TimMoore/restore-v1.5-index

Restore v1.5 index page