Fix for "uninitialized constant Sinatra::Sprockets::Manifest"

Fixes:

    $ rake assets:precompile
    ** Invoke assets:precompile (first_time)
    ** Execute assets:precompile
    rake aborted!
    uninitialized constant Sinatra::Sprockets::Manifest
    …/gems/sinatra-asset-pipeline-0.2.0/lib/sinatra/asset_pipeline/task.rb:13:in `block (2 levels) in initialize'