SPARK-1658: Correctly identify if maven is installed and working

The current test is checking the exit code of "tail" rather than "mvn".
This new check will make sure that mvn is installed and was able to
execute the "version command".

Author: Rahul Singhal <rahul.singhal@guavus.com>

Closes #580 from rahulsinghaliitd/SPARK-1658 and squashes the following commits:

83c0313 [Rahul Singhal] SPARK-1658: Correctly identify if maven is installed and working
bf821b9 [Rahul Singhal] SPARK-1658: Correctly identify if maven is installed and working
(cherry picked from commit e97a2e6717e75c70c5474f2d211682565909e557)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>