[ENSCORESW-3077] Travis: Unify configuration of Slack notifications
[ENSCORESW-3077] Travis: Unify configuration of Slack notifications

...but keep notifying #ehive-commits.

Ensembl/ensembl-hive,Ensembl/ensembl-hive,Ensembl/ensembl-hive,Ensembl/ensembl-hive