[build] Actively avoid travis "no output" timeout for 15min

This is done by having an echo on the background with 3x5min sleeps.
The echo is killed if the rest of the travis_script takes less time
to complete.

Solution inspired from this issue comment while looking at travis_wait:
travis-ci/travis-ci#4190 (comment)

travis-build.sh

@@ -2,6 +2,12 @@
 
 set -ev
 
+for i in 1 2 3
+do
+    sleep 300;
+    echo "=====[ $((i * 5))min, still running ]=====";
+done &
+
 if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
     COMMIT_RANGE="FETCH_HEAD..$TRAVIS_BRANCH"
     echo "travis PR #$TRAVIS_PULL_REQUEST build, looking at files in $COMMIT_RANGE"
@@ -39,4 +45,6 @@ else
     fi
 fi
 
+kill %1
+
 exit 0;