add Mito and re-add Clack-Errors packages to .asd file.

Mito is a new package introduced in Chapter 4. Clack-Errors was first
added in Chapter 1 but I, some how, managed to remove it. I am
re-adding here and committing it so I have a log of me adding it --
and remove doubt on me accidentally thinking I added. Which, to be
honest, is something I cannot rule out.

rails-to-caveman.asd

@@ -1,9 +1,10 @@
 (defsystem "rails-to-caveman"
   :version "0.1.0"
   :author "Craig Oates"
-  :license ""
+  :license "MIT"
   :depends-on ("clack"
                "lack"
+	       "clack-errors" ; <--- Added in Chapter 1.
                "caveman2"
                "envy"
                "cl-ppcre"
@@ -17,7 +18,8 @@
 
                ;; for DB
                "datafly"
-               "sxql")
+               "sxql"
+	       "mito" ; <--- Added in Chapter 4.
   :components ((:module "src"
                 :components
                 ((:file "main" :depends-on ("config" "view" "db"))