Update get-stack.sh for centos 6
Update get-stack.sh for centos 6

If a CentOS user has command lsb_release, the VERSION variable for distro_info() will be "6.xx", not just "6".  Since try_lsb() is the first test done in distro_info(), if the user has lsb_release, and any version of CentOS 6, the case statement will not point the user to the correct gmp linked binary.

NOTE:  I can't perform the test for stack 1.9 because an update to the sudocmd() function fails... that will also need to be fixed for stack 1.9
juhp/stack,juhp/stack