temporarily allow ruby-head to fail in CI

Travis has been erroring in the last days for ruby-head. It timeouts
while running

    ruby-head - #importing gemset /home/travis/.rvm/gemsets/global.gems

which seems unrelated to Zeitwerk. I have reported it.

Let's allow failures on ruby-head while this is sorted out, the red
badge in the home page is misleading.

.travis.yml

@@ -13,5 +13,6 @@ rvm:
   - truffleruby
 matrix:
   allow_failures:
+    - rvm: ruby-head
     - rvm: jruby-head
     - rvm: truffleruby