Fixed built-in function checks ACID

ACID tests for the existence of the tf and chirp function
using `exist(XX,'builtin')`. This doesn't work on MATLAB 2013b
and possibly also other versions.

Instead, the existence of the function is checked using `which`.
This causes the corresponding test cases to be run instead of skipped
Merge pull request #274 from egeerardyn/fixACID

Fixed built-in function checks ACID