Restores build to working condition and other administrativa

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Binaries for Scala.js 0.6.21 on Scala 2.12.6 are gone from the central repo (not sure how that happens...), which is the primary trigger for this PR.

Other minor changes include:

• Support JDK 9+ builds
• Set up Travis to build on JDK 8 and 11 (as 10 is deprecated)
• Copyright notice updates
• Scala.js version badge on index