Bump sbt to 1.2.6; update dependencies

[ucbjrl]

Type of change: other enhancement

Impact: no functional change

Development Phase: implementation

Release Notes
Bump sbt version to 1.2.6
Update dependency versions

[chick]

LGTM

[ucbjrl]

Why the explicit reference to junit?

I was running into problems with transitive dependencies, specifically:
Symbol 'term org.junit' is missing from the classpath.
I suspect this is due to interaction between the latest version of sbt and org.scala-sbt.ivy. It may be because junit is special - it's pure Java and independent of the Scala compiler. Forcing a more recent version of junit seemed to resolve those problems.

[jackkoenig]

@ucbjrl can you add a comment about that? That is a bit weird...

[ucbjrl]

The:
Symbol 'term org.junit' is missing from the classpath
occurred when running tests under Scala 2.11.12.

sbt test

and

sbt "++2.12.7" test

both work correctly. But

sbt +test

and

sbt "++2.11.12" test

both fail with

[error] /nscratch/ucbjrl/firrtl/src/test/scala/firrtlTests/FirrtlSpec.scala:193:18: Symbol 'term org.junit' is missing from the classpath.
[error] This symbol is required by 'class org.scalatest.junit.JUnitRunner'.
[error] Make sure that term junit is in your classpath and check for conflicting dependencies with `-Ylog-classpath`.
[error] A full rebuild may help if 'JUnitRunner.class' was compiled against an incompatible version of org.
[error]   implicit class TestingFunctionsOnCircuit(val circuit: Circuit) extends AnyVal {
[error]                  ^
[warn] one warning found
[error] one error found
[error] (Test / compileIncremental) Compilation failed

when compiling for 2.11.12. Adding a specific dependency on junit in build.sbt seems to alleviate the issue.

NOTE: mill has a similar problem, so it may be a ammonite issue.

[jackkoenig]

Would you mind adding a short explanation as a comment in the build.sbt?