Add support for Bash 3.0

[Chemaclass]

Right now, bashunit requires/support Bash 3.2 and newer. However, why not expand the compatibility to start supporting from Bash 3.0 and so give that old code the possibility to write tests as well?

The reason why is currently 3.2 is because my own MacOS got this version by default. This has prevented me to use modern Bash syntax, therefore adapting to 3.0 might not be that difficult and we could give support to more developers out of the box.