InspIRCd: fix various audit warnings.

Fixed the following `brew audit --strict` warnings:

 * A `test do` test block should be added
 * `require 'formula'` is now unnecessary
 * Use `system 'make", "install'` instead of `system 'make install'` 

I have also converted the formula to use double quotes everywhere as this seems to be what other formulae are switching to.