Simpler logic to category patch

Replace single line with cleaner, more readable Ruby.

Rakefile

@@ -47,7 +47,7 @@ task :post do
   title = ENV["title"] || "new-post"
   tags = ENV["tags"] || "[]"
   category = ENV["category"] || ""
-  category = category.empty? ? "" : "\"#{category.gsub(/-/,' ')}\""
+  category = "\"#{category.gsub(/-/,' ')}\"" if !category.empty?
   slug = title.downcase.strip.gsub(' ', '-').gsub(/[^\w-]/, '')
   begin
     date = (ENV['date'] ? Time.parse(ENV['date']) : Time.now).strftime('%Y-%m-%d')