Update brew instructions to build Boost from HEAD

Previously, when trying to build master or v0.9.0, compilation on OS X would
fail [1]. This commit resolves these errors by building Boost from source.
Building from HEAD removes the guarantee of a repeatable build, but this
appears to be a necessity until the next version of Boost is released.

At the time of this writing, the current version of Boost is 1.55.0 [2]. 
Presumably, the next release will include the changes now on HEAD, and once
that new version has been made available in Homebrew [3], this commit should
be reverted.

[1]: https://gist.github.com/9704573
[2]: http://www.boost.org/users/history
[3]: https://github.com/Homebrew/homebrew/commits/master/Library/Formula/boost.rb