bootstrap: warn user if rocks-devel is too old

with this new binaryless repo you need rocks-devel-6.2 to be
able to download the binaries

I used the following script to modify the bootstrap.sh

awk '/. \$ROLLSROOT\/etc\/bootstrap-functions.sh/{print "if [ ! -f \"$ROLLSROOT/../../bin/get_sources.sh\" ]; then\n\techo \"To compile this roll on Rocks 6.1.1 or older you need to install a newer rocks-devel rpm.\nInstall it with:\nrpm -Uvh https://googledrive.com/host/0B0LD0shfkvCRRGtadUFTQkhoZWs/rocks-devel-6.2-3.x86_64.rpm\nIf you need an older version of this roll you can get it from:\nhttps://github.com/rocksclusters-attic\"\n\texit 1\nfi\n"}1' bootstrap.sh > tmp
  cp tmp bootstrap.sh