[build] Travis uses sauce Labs limited to 5 concurrent jobs

[ignasi35]

When migrating to travis-ci.com we increased the number of workers to up-to 13. This surfaced an issue in the build:

• Sauce Labs limits to 5 the number of concurrent jobs so it's not uncommon to see job failures when all the Travis worker pool is available.

This is currently fixed using https://blog.travis-ci.com/2014-07-18-per-repository-concurrency-setting but that's not git tracked or explicit in .travis.yml.

[ignasi35]

An option is to merge scripts into one fat script doing test, validateCode, etc...

Another option is to split the build into stages of 4 jobs each (each stage is a JDK+SCALA_VERSION combination).

[ignasi35]

I don't think we need the Sauce Labs add-on on every job (I don't think it's used in validateCode, for example) so maybe setting the add-on conditionally could be useful too.

[xuwei-k]

5b35f53

[mkurz]

How was that sauce labs actually used? For browser testing I guess, bit I don't think we need it anymore...