Travis: print spaces every minute to fix build

Package wxwidgets is downloaded for more than 10 minutes,
Travis failed because of no output for 10 minutes:
https://travis-ci.org/mxe/mxe/builds/205649309

.travis.yml

@@ -4,6 +4,9 @@ sudo: false
 script:
   - make docs/build-matrix.html docs/packages.json OS_SHORT_NAME=disable-native-plugins
   - if [ "$GH_TOKEN" != "" ]; then ./tools/travis-push.sh; fi
+  # Package wxwidgets is downloaded for more than 10 minutes,
+  # Travis failed because of no output for 10 minutes.
+  - while true; do sleep 60; echo -n ' '; done &
   - make download -j 6 -k MXE_PLUGIN_DIRS="$(./tools/plugins-with-additional-packages.sh)"
 
 env: