Pare down Travis build matrix

[ashawley]

Build count increased a lot in #223, and the run time increased as well.

Want to investigate if there are certain builds that are less of a priority.

I believe this is the current build matrix:

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

Out of a 27 possible (3 choose 1, times 3) there are currently 17, and the rest are excluded.