libffi formula needs to be deparallelized

libffi would only successfully install on my machine 10% of the time. I ran many tests uninstalling and reinstalling libffi to confirm this.

Failure (~90% of attempts) : https://gist.github.com/KendallPark/5389215#file-brew-boxen-install-debug-verbose-libffi-failure-L165-L166
Success (~10% of attempts) : https://gist.github.com/KendallPark/5389203#file-brew-boxen-install-libffi-debug-verbose-success-L165-L166

after adding "ENV.deparallelize" the install worked 100% of the time. 

```sh
==> Configuration
HOMEBREW_VERSION: 0.9.4
HEAD: 552074b5a345e1041eef866bf219edbf053b0825
HOMEBREW_PREFIX: /opt/boxen/homebrew
CPU: 8-core 64-bit ivybridge
OS X: 10.8.3-x86_64
Xcode: 4.6.1
CLT: 4.6.0.0.1.1362189000
X11: 2.7.4 => /opt/X11
```