Ref https://github.com/sbt/librarymanagement/pull/280
This is to workaround bincompat error detected by sbt community build.
```
[cats] [error] java.lang.NoSuchMethodError: sbt.librarymanagement.CrossVersion$.Disabled()Lsbt/librarymanagement/Disabled$;
```
Fixes#274
In #249 parallel download switched to using its own thread pool.
It could potentially lead to unbounded download if nobody throttled.
This works around the issue by fixing the number of thread to 6, which is a common per-host max connection count.
Rather than
[warn] There may be incompatibilities among your library dependencies.
[warn] Run 'evicted' to see detailed eviction warnings
[info] Done updating.
[warn] There may be incompatibilities among your library dependencies.
[warn] Run 'evicted' to see detailed eviction warnings
[info] Done updating.
[warn] There may be incompatibilities among your library dependencies.
[warn] Run 'evicted' to see detailed eviction warnings
[info] Done updating.
[warn] There may be incompatibilities among your library dependencies.
[warn] Run 'evicted' to see detailed eviction warnings
I would get:
[warn] There may be incompatibilities among your library dependencies; run 'evicted' to see detailed eviction warnings
[info] Done updating.
[warn] There may be incompatibilities among your library dependencies; run 'evicted' to see detailed eviction warnings
[info] Done updating.
[warn] There may be incompatibilities among your library dependencies; run 'evicted' to see detailed eviction warnings
[info] Done updating.
[warn] There may be incompatibilities among your library dependencies; run 'evicted' to see detailed eviction warnings
which is a touch better. (IMO!)