Correctly cache ccache build dir

According to
https://docs.travis-ci.com/user/caching/#ccache-cache
We typically would want to do `cache: ccache` but since we are also
caching other dirs we have this option set manually.

However it looks like the ccache dir was not set to be relative to home,
which meant we were not getting any of the ccache cache wins across
builds (which can be seen in how long it takes to rebuild v8worker2 each
time)

.travis.yml

@@ -2,8 +2,8 @@ language: c++
 cache:
   ccache: true
   directories:
-   - .cache/
-   - node_modules/
+   - $HOME/.ccache
+   - node_modules
    - $V8WORKER2_OUT_PATH
 env:
   global: