[Hot Fix][Streaming] Fix WriteAheadLogBackedBlockRDDSuite

```WriteAheadLogBackedBlockRDDSuite``` failed due to the same random file name(https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/24456/consoleFull)
```
/tmp/1418645532799-0/??????????
[info] - Read data available only in write ahead log, and test storing in block manager *** FAILED *** (2 milliseconds)
```
Using another way to produce random file name(http://alvinalexander.com/scala/creating-random-strings-in-scala).