Fix to support LLVM/ruby that ships with Xcode 5.1

https://github.com/boxen/boxen-web/issues/51

- It currently checks for the exact version of Xcode that introduced the LLVM/Ruby issue. Didn't want to be too overzealous in matching for versions newer than this since the problem is dealt with in newer versions of Ruby.
- Shell nitpicking: exported the `ARCHFLAGS` option so it would be in effect for the entire initial run of Boxen, wasn't sure offhand if setting it in the command only was preferable/fine (e.g. `ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future script/boxen`).