Fix: use Bash for all shell scripts

This ensures that echo's `-e` option is recognized and
does the right thing since `echo` is a shell built-in, and
its behaviour is not constant from one shell to the other.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Merge pull request #29 from eepp/use-bash

Fix: use Bash for all shell scripts