Fixes #68 non-existing templates.js causes issue with sinatra-assetpack

sinatra-assetpack is not handling the non-existing templates.js asset specified by frontend.rb. This issue only appears when not in development thus people normally find it only once they are trying to deploy. I am not sure adding the empty file is the best way though.

There isn't an exact issue about this at sinatra-assetpack but the following issue and pull request mention the problem of dealing with non-existing dynamic assets: rstacruz/sinatra-assetpack#86 and https://github.com/rstacruz/sinatra-assetpack/pull/91