### What changes were proposed in this pull request?
Support Spark2.4 with Scala2.12 in `sbt.md`. Meanwhile, the CI workflow adds the test for Spark2.4 and Scala2.12.
Follow up #2344.
### Why are the changes needed?
Spark2.4 with Scala2.12 is supported.
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
CI.
Closes#2345 from SteNicholas/CELEBORN-1298.
Authored-by: SteNicholas <programgeek@163.com>
Signed-off-by: SteNicholas <programgeek@163.com>