Scala version detection is broken for 2.11.12

[gzm0]

Surfaced by #30.

[gzm0]

Symptom:

This bundle of Scala.js CLI is for 2.11. Your scala version is 1.8.0
2.11.12!

It seems that 1.8.0 is a Java version that is also in the stdout. I cannot reproduce this locally.

[gzm0]

I suspect that something sets java's -showversion flag.