Fix the error about pagerank when $COMPRESS -eq 0 

When $COMPRESS -eq 0, COMPRESS_OPT is an unbound variable. And '$HADOOP_EXECUTABLE jar ${DATATOOLS} HiBench.DataGen ${OPTION} ${COMPRESS_OPT}' can not run.
"-c false" can not be used as COMPRESS_OPT

 COMPRESS_OPT="-c false" is illegal for pagerank/bin/prepare.sh
Merge pull request #60 from Earne/master

Fix the error about pagerank when $COMPRESS -eq 0