Set a preceding slash in the assets prefix

None of our apps configure the 'config.assets.prefix' variable with
a preceding slash, but the Rails configuration guide suggests we
should do [1].

The previous change causes asset URLs to omit any slash between the
hostname and path at all, so this seems like the most appropriate
way to fix this problem.

[1] http://guides.rubyonrails.org/asset_pipeline.html#changing-the-assets-path
Merge pull request #4 from alphagov/asset-configuration-mk2

Set a preceding slash in the assets prefix