Try caching both the debug and release ccache dirs

My theory is that specifying the separate cache directories
interacts badly with travis-ci.  I think it will either delete
any cache directories that look like they are no longer cached
(ie when it finishes the debug build it will delete the release
ccache dir since it looks unused), or maybe it keeps them around
but only checks the most recent build for a cache hit (ie the
debug build might check and only find ccache_release).

So anyway, try always specifying both directories.  Another
option is to do the simpler thing and use a single directory for
both caches, but I want to only change one thing at a time.

.travis.yml

@@ -21,7 +21,8 @@ sudo: false
 
 cache:
   directories:
-    - ${CCACHE_DIR}
+    - $HOME/.ccache_debug
+    - $HOME/.ccache_release
 
 addons:
   apt: