Update .travis.yml to ajust to new release cadence

[jsvd]

No description provided.

[donoghuc]

Hmm looks like for 8.future we are missing this transformation:

#1 [logstash internal] load build definition from Dockerfile
#1 transferring dockerfile: 1.94kB done
#1 WARN: InvalidDefaultArgInFrom: Default value for ARG docker.elastic.co/logstash/logstash${DISTRIBUTION_SUFFIX}:${ELASTIC_STACK_VERSION} results in empty or invalid base image name (line 3)
#1 DONE 0.0s
#2 [logstash internal] load metadata for docker.elastic.co/logstash/logstash:8.future
#2 ERROR: docker.elastic.co/logstash/logstash:8.future: not found

https://github.com/logstash-plugins/.ci/blob/670ab81da4e9e6878fafaff72cf79df3bb8498e3/docker-setup.sh#L47-L57

[jsvd]

allowing a 8.future stack version to not exist is not ideal but acceptable, so it should not block the build from being green: https://github.com/logstash-plugins/.ci/pull/68/files

[donoghuc]

Agree. Looks like that PR is merged. If you mark this as "ready for review" instead of "draft" will that trigger a fresh travis run? If we go green on that we should be g2g

[jsvd]

I manually requested https://app.travis-ci.com/github/logstash-plugins/logstash-filter-geoip/builds/273267562, let's see..

[jsvd]

@donoghuc I've simplified the travis.yml by only making it add the extra oss tests, leaving the default distribution tests to be read by the shared .ci definition. everything's green now.

[donoghuc]

+1 to the simplified layout.