[SPARK-1841]: update scalatest to version 2.1.5

Author: witgo <witgo@qq.com>

Closes #713 from witgo/scalatest and squashes the following commits:

b627a6a [witgo] merge master
51fb3d6 [witgo] merge master
3771474 [witgo] fix RDDSuite
996d6f9 [witgo] fix TimeStampedWeakValueHashMap test
9dfa4e7 [witgo] merge bug
1479b22 [witgo] merge master
29b9194 [witgo] fix code style
022a7a2 [witgo] fix test dependency
a52c0fa [witgo] fix test dependency
cd8f59d [witgo] Merge branch 'master' of https://github.com/apache/spark into scalatest
046540d [witgo] fix RDDSuite.scala
2c543b9 [witgo] fix ReplSuite.scala
c458928 [witgo] update scalatest to version 2.1.5