.travis.yml: show free disk space at end of run

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200501111505.4225-2-alex.bennee@linaro.org>
diff --git a/.travis.yml b/.travis.yml
index 2fd63ec..a4c3c6c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -113,6 +113,7 @@
         $(exit $BUILD_RC);
     fi
 after_script:
+  - df -h
   - if command -v ccache ; then ccache --show-stats ; fi