Update mruby-head installed on Travis CI

This way other projects can test against it.

.travis.yml

@@ -5,3 +5,11 @@ install:
 
 env: MRUBY_CONFIG=travis_config.rb
 script: "./minirake all test"
+
+notifications:
+  # Update mruby-head installed on Travis CI so other projects can test against it.
+  webhooks:
+    urls:
+      - "https://rubies.travis-ci.org/rebuild/mruby-head"
+    on_success: always
+    on_failure: never