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

Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200501111505.4225-2-alex.bennee@linaro.org>

.travis.yml

@@ -113,6 +113,7 @@ script:
         $(exit $BUILD_RC);
     fi
 after_script:
+  - df -h
   - if command -v ccache ; then ccache --show-stats ; fi