[BUGFIX] Fix for sbt/sbt script SPARK_HOME setting

In some environments, this command

    export SPARK_HOME=$(cd "$(dirname $0)/.."; pwd)

echoes two paths, one by the "cd ..", and one by the "pwd". Note the resulting
erroneous -jar paths below:

    ctn@ubuntu:~/src/spark$ sbt/sbt
    + EXTRA_ARGS=
    + '[' '' '!=' '' ']'
    +++ dirname sbt/sbt
    ++ cd sbt/..
    ++ pwd
    + export 'SPARK_HOME=/home/ctn/src/spark
    /home/ctn/src/spark'
    + SPARK_HOME='/home/ctn/src/spark
    /home/ctn/src/spark'
    + export SPARK_TESTING=1
    + SPARK_TESTING=1
    + java -Xmx1200m -XX:MaxPermSize=350m -XX:ReservedCodeCacheSize=128m -jar /home/ctn/src/spark /home/ctn/src/spark/sbt/sbt-launch-0.11.3-2.jar
    Error: Invalid or corrupt jarfile /home/ctn/src/spark

Committer: ctn <ctn@adatao.com>

On branch master
Changes to be committed:

- Send output of the "cd .." part to /dev/null
	modified:   sbt/sbt