Remove compiling with objc gc

I'm not sure of the full implications of this, but I had to remove gc to compile nu. I'm running OS X 10.9.2 with xcode 5.1 and command line tools 5.1.0.0.1.1393561416.

Here's the error:

```sh
$ ./mininush tools/nuke
Using Nukefile with target default.
nuke: running in /Users/sj26/Projects/nu
nuke: /usr/bin/clang -Wall -g -fPIC -O2 -DMACOSX   -DHAVE_CONFIG_H -fobjc-exceptions -fobjc-gc -arch x86_64  -I/usr/include  -I./include  -I./include/Nu  -I/usr/local/include  -I/usr/include/ffi -c -o build/x86_64/Nu.o objc/Nu.m
error: garbage collection is no longer supported
nuke: terminating on command error (return code 1)
```