Removes cache

Which is probably useless anyway. But this "canary in the mine" test
is fast anyway, and gives Shippable a good use, so closes #1931. We
might leave it thus for a while, and maybe incorporate it to Travis
later on, if possible (refs #1946).

Author: JJ

shippable.yml

@@ -1,7 +1,5 @@
 build:
   cache: true
-  cache_dir_list:
-    - $SHIPPABLE_BUILD_DIR/.perl6
 
   ci:
     - apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 379CE192D401AB61