Update .travis.yml to account for 9.x / main

[jsvd]

Tests were failing in this plugin, this PR fixes that by:

• Allowing for 9.next to not exist but still mark the build as green
• Fixing a test related to metrics of a partially failed bulk by writing to a random index instead of the logs- stream (which has failure store enabled by default)
• Adding main to secure integration testing.

[donoghuc]

I think i may have missed a 9.next version in this elastic/logstash#17868 as it would always be main for the near future?

[jsvd]

next is a moniker we came up with to reference that short lived situation between the branching off of a new 9.x minor from main until its GA. e.g. once we hit 9.2.0 feature freeze we'll branch 9.2 from main, and have to deal with main, 9.2, 9.1 and 9.0 until 9.2 GA, where 9.2 becomes .current, 9.1 previous, and drop 9.0.
For this reason it's fine that next is often enough not real until it is, so we excuse its absence.

[donoghuc]

Changes look good.

I see

Fixing a test related to metrics of a partially failed bulk by writing to a random index instead of the logs- stream (which has failure store enabled by default)

in the PR description which would help an archeologist figure out the index configuration, but a comment may help (thinking of a future test reader wondering why there is a random index that is never explicitly queried for by name).

I was planning on discussion #1220 (comment) at sync.