Update build_linux.sh

Adds compatibility to Debian 6 squeeze. Prevents running this script by "SH" instead of "BASH" during installation of "rmr2" on Debian 6. I agree to assign copyright of that code to Revolution Analytics Inc., on the condition that it releases that code under the Apache 2.0 license.
Merge pull request #75 from khharut/patch-5

Shebang not honored on Debian 6