Fix Gitter webhook call from Travis
Fix Gitter webhook call from Travis

Encrypted blob created with:

    travis encrypt "https://webhooks.gitter.im/e/05****************16" -r http4s/http4s

Adapted based on advice here:
https://docs.travis-ci.com/user/notifications/#Configuring-slack-notifications

Will need to run through Travis to confirm it works as expected.

aeons/http4s,reactormonk/http4s,ChristopherDavenport/http4s,aeons/http4s,ChristopherDavenport/http4s,reactormonk/http4s,ZizhengTai/http4s,ChristopherDavenport/http4s,rossabaker/http4s,http4s/http4s,rossabaker/http4s,aeons/http4s,reactormonk/http4s,ZizhengTai/http4s,ZizhengTai/http4s