Travis-CI config doesn't work from forks

[SethTisue]

some contributors might appreciate being able to run PR validation in their forks, for work that isn't ready for the main scala/Scala PR queue

@dwijnand made a brief initial stab at scala/scala#8665 but quickly realized it needed more work

[lrytz]

Somewhat related, here is a script that I wrote at some point to do a full bootstrap build locally, using the scripts in the repo: lrytz/scala@25d246d. (For local development and testing, I do it differently though. I basically do sbt setupPublishCoreNonOpt publishLocal and then sbt -Dstarr.version=..... dist/mkQuick.)

Another issue we might run into is time limits. Travis by default gives 50 minutes for public repositories. We got a free upgrade for scala/scala (I think to 90 minutes?) at some point, but that likely doesn't apply to forks. PR builds on travis ususally take 1h to 1h15m.

[dwijnand]

I also don't think (and highly doubt) forks reused scala/scala's cache, so it's likely longer.

I think it would be fine if forks only ran non-bootstrapped.

[lrytz]

The bootstrap doesn't take much time, it's the test suite.

[dwijnand]

Oh I thought tests were run twice, bootstrapped and non-bootstrapped.