Implemented Lua Unit Testing Support

I've implemented CorsixTH support for the Busted Lua unit testing
framework by adding 2 statements to the travisCI configuration file (.travis.yml).

The first statement uses luarocks to install Busted on the travisCI worker virtual machine and then the second statement uses Busted to test all the unit test scripts in: CorsixTH/Lua/unit_tests and its sub directories.