#1541. Use HTTPS for sbt plugin repository

#1541. This is the last of builtin repo that's HTTP. (Except for Java.net Maven 1 repo that we just deprecated).

```
[info] downloading https://repo.scala-sbt.org/scalasbt/sbt-plugin-releases/com.eed3si9n/sbt-assembly/scala_2.10/sbt_0.13/0.11.1/jars/sbt-assembly.jar ...
[info]  [SUCCESSFUL ] com.eed3si9n#sbt-assembly;0.11.1!sbt-assembly.jar (1544ms)
```