Set always_build on Berkshelf

Berkshelf is pointing to master of berkshelf-api for tests. We need to set berkshelf to always_build otherwise we get errors similar to this when running it:

/opt/chefdk/embedded/lib/ruby/gems/2.1.0/gems/bundler-1.5.2/lib/bundler/spec_set.rb:92:in `block in materialize': Could not find berkshelf-api-1.2.1 in any of the sources (Bundler::GemNotFound)

@opscode/client-eng