Updated runreef.psm1

 * When on a machine with no YARN install ('$HADOOP_HOME' isn't set), it will work, assuming that the main class loaded does.
 * When the path to $JAVA_HOME contains spaces (e.g. 'C:\Program Files\java\...'), it will now work.
Merge pull request #772 from Microsoft-CISL/weimer_runreef

Updated runreef.psm1