Upgrade CI build

[diesalbla]

Upgrade Ruby version to 2.3.0.
Add othe JDKs
Upgrade Scala version 2.12 to 2.12.8

[codecov]

Codecov Report

Merging #173 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #173   +/-   ##
======================================
  Coverage    95.2%   95.2%           
======================================
  Files           6       6           
  Lines         167     167           
  Branches        7       7           
======================================
  Hits          159     159           
  Misses          8       8

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d9af77c...ff6913d. Read the comment docs.

[codecov]

Codecov Report

Merging #173 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #173   +/-   ##
======================================
  Coverage    95.2%   95.2%           
======================================
  Files           6       6           
  Lines         167     167           
  Branches        7       7           
======================================
  Hits          159     159           
  Misses          8       8

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d9af77c...db69d83. Read the comment docs.

[purrgrammer]

It seems that for some builds sbt cannot find the Scalajs compiler version, see this build log.

[juanpedromoreno]

I think that you can add addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.26") to the plugins.sbt file, so you can use Scala 2.12.8.