Update postcss config path to match new structure

Author: EricGusmao

Diff for: lib/tailwindcss/commands.rb

@@ -14,7 +14,7 @@ def compile_command(debug: false, **kwargs)
 
         command << "--minify" unless (debug || rails_css_compressor?)
 
-        postcss_path = rails_root.join("config/postcss.config.js")
+        postcss_path = rails_root.join("postcss.config.js")
         command += ["--postcss", postcss_path.to_s] if File.exist?(postcss_path)
 
         command