(maint) allow running w/ `RAILS_ENV=production`

Prior to this commit, the rails 3 version of dashboard would not run in
production because of two problems:

 1. bundler didn't know to look for the `haml` gem outside the `:assets`
group
 2. the production config explicitly didn't serve static assets (like
images)

This commit adds the `haml` gem, already a dependency of the existing
`haml-rails` gem in the `:assets` group, but outside the `:assets` group for
availability after assets have been precompiled. This commit also allows
for serving static assets.