Add typing/gradual to track.rb

Finishes implementation with a filter addition (linter + docs already done).

Cf. exercism/docs#581 and http://forum.exercism.org/t/adding-typing-gradual-to-the-accepted-tags-list/17127/13

Author: codingthat

app/models/track.rb

@@ -153,6 +153,7 @@ def github_team_name = slug
     typing: {
       static: "Static",
       dynamic: "Dynamic",
+      gradual: "Gradual",
       strong: "Strong",
       weak: "Weak"
     },