Fixed Pow installation on Yosemite based on #453

This is fix of basecamp/pow#452 based on @koenpunt's PR #453.

### How to install
```sh
brew update
brew install node
git clone git@github.com:asux/pow.git ~/Projects/pow
cd ~/Projects/pow
./build
./install
```

You should revert 4f4a031 before release of new tarball.