Reduce Travis CI Slack notification frequency
Reduce Travis CI Slack notification frequency

Per https://docs.travis-ci.com/user/notifications/#Configuring-Slack-notifications
the default setting for Travis Slack notifications is to send a message
on every successful build. This commit changes this frequency, such that
notifications are always sent when builds fail and only sent when builds succeed
after a previous failure.

bisq-network/exchange,bitsquare/bitsquare,bitsquare/bitsquare,bisq-network/exchange