Convert all hashbangs to point to bash, not whatever happens to be the distro's default shell

tl;dr: This makes your build system work easily on any Debian-based systems.

Debian-based systems' (Debian, Ubuntu, Mint, etc) /bin/sh leads to dash which does NOT support the Bashisms you are using in all your scripts (chiefly, double-square-bracket comparisons).

Any sane system will place bash in /bin/bash, or at least provide a symlink at /bin/bash to wherever bash actually is located. Therefore, this change should not harm any existing systems.