Increase stack size.

With -Xlog-implicits I was able to track down that the inductive
derivation for the case-app parser was failing with a stack overflow
```
[info] hasMatchingSymbol reported error: exception during macro expansion:
[info] java.lang.StackOverflowError
[info]     at scala.tools.nsc.typechecker.StdAttachments.isMacroImplRef(StdAttachments.scala:153)
[info]     at scala.tools.nsc.typechecker.StdAttachments.isMacroImplRef$(StdAttachments.scala:153)
[info]     at scala.tools.nsc.Global$$anon$1.isMacroImplRef(Global.scala:458)
[info]     at scala.tools.nsc.typechecker.Typers$Typer.adapt(Typers.scala:1175)
```

This could offer an explanation why 2.12.4 is a regression, since the
stack size might have changed.

.jvmopts

@@ -1,4 +1,5 @@
 -Dfile.encoding=UTF8
+-Xss8m
 -Xms1G
 -Xmx6G
 -XX:ReservedCodeCacheSize=250M