SPARK-2482: Resolve sbt warnings during build

At the same time, import the `scala.language.postfixOps` and ` org.scalatest.time.SpanSugar._` cause `scala.language.postfixOps` doesn't work

Author: witgo <witgo@qq.com>

Closes #1330 from witgo/sbt_warnings3 and squashes the following commits:

179ba61 [witgo] Resolve sbt warnings during build