Fewer Travis builds

[ashawley]

Fixes #226.

New matrix is:

Java	Scala	Scala.js
openjdk6	2.11.12
openjdk6	2.11.12	0.6.23
openjdk6	2.11.12	1.0.0-M3
oraclejdk8	2.12.6
oraclejdk8	2.12.6	0.6.23
oraclejdk8	2.12.6	1.0.0-M3
oraclejdk8	2.13.0-M3
oraclejdk8	2.13.0-M3	0.6.23
oraclejdk8	2.13.0-M3	1.0.0-M3
oraclejdk9	2.12.6
oraclejdk9	2.13.0-M3

[ashawley]

This brings the the build time in Travis back to 10 minutes.

Do we need to build Scala.js in every version of Scala for publishing purposes?

[lrytz]

We should continue to release for both Scala.js versions on 2.11 (like we do for 2.12, and would do for 2.13 if both Scala.js were available). The rest looks good to me. I'm also fine to skip Scala.js entirely on Java 9.

[ashawley]

Ok, I'll try to add the scalajs builds back for all the Scala versions.

If I can't figure out the exclusion rules, then maybe I'll see if it's easier to write this as includes.

[SethTisue]

@ashawley shall we merge...?