Store ccache directory explicitly in Travis. (#10119)

* Store ccache directory explicitly in Travis.

Despite we started using ccache on `develop`, it seems the cache itself
is not stored from one CI build to the next. This might be du to the
fact that our language on Travis is Python and not C nor C++.

Hence here we store the ccache directory explicitly.

.travis.yml

@@ -143,11 +143,14 @@ addons:
       - perl
       - perl-base
 
+# ~/.ccache needs to be cached directly as Travis is not taking care of it
+# (possibly because we use 'language: python' and not 'language: c')
 cache:
   pip: true
   ccache: true
   directories:
     - ~/.mirror
+    - ~/.ccache
 
 # Work around Travis's lack of support for Python on OSX
 before_install: